• If you plan to purchase a semi-automatic pistol, buy a BB pistol like this one (I’ve bought three of them so far, and highly recommend them) or an Airsoft pistol like this one (which I also know from personal experience).
• Of course, you don’t have to select one of those – there are many other choices out there.  Do your own research.  Buy also enough air cylinders (for gas-operated pistols) and ammo (BB’s or airsoft pellets) for at least 1,000 rounds of training – I personally prefer 5,000.
• If you plan to purchase a double-action revolver, buy a similar BB revolver or Airsoft equivalent for training purposes, again with enough ammo as described above.  (I don’t have personal experience with either model, so I can’t make any promises about them.)
• If you want to buy a pump-action shotgun, try this as a training equivalent.
• If you’re after a lever-action rifle, how about this one?
• If you haven’t yet decided what type of weapon you want to buy, start training anyway!  Buy a basic BB or Airsoft handgun or long gun and put it to good use.  You’ll find that basic shooting skills translate relatively easily between weapon types.  While you’re learning the basics, you can save your money to buy the real thing.

You can find equivalent BB or Airsoft clones of many regular firearms, and typically buy one of them and air cartridges and ammunition for well under $100 total.  Use them, plus instructional video clips from YouTube (ignore the ‘tacticool’ over-the-top videos with rock music soundtracks – look for sober, sensible presentations without hype) and the help of friends who know what they’re doing, to get a basic idea of how to aim and fire a weapon.  Concentrate on slow, accurate shooting at first, until you can put all your rounds into the center of a target (I use the cheapest grade of 8″ paper, plastic or foam disposable plates).  Start at close range (10-15 feet) and extend it as you get better, until you can group 10 rounds inside 4″-6″ at 15-20 yards on demand.  (This assumes your training weapon is capable of such accuracy, of course.)
Once you’ve learned the basics of accurate shooting, try to shoot faster without losing accuracy.  Defensive shooting is typically at close range, so set up small targets (e.g. tennis balls to start, later progressing to even smaller table-tennis or squash balls) at varying distances (10-15 feet at first, moving out in stages to 15-20 yards) and try to hit them.  I’ve trained several wheelchair-bound shooters to hit a rolling squash or table-tennis ball (or a target of similar size) with at least 7 out of 10 rounds from a handgun at distances of 10 to 20 feet, all within 5 to 6 seconds.  That may sound daunting, but after a few thousand rounds of practice, starting with a BB pistol and progressing to a .22LR firearm, they didn’t find this too difficult.  As I said to them, if they can hit a small, moving target that often at that range, they should have little trouble hitting someone threatening their safety!
I highly recommend taking a basic training course from a qualified instructor if possible.  Many shooting ranges offer NRA courses, and some police and sheriff’s departments offer firearms training for citizens.  They’ll probably expect you to use a real gun when you attend the course, rather than a BB or Airsoft ‘toy’, but they may be able to arrange a ‘loaner’ firearm for you.  There’s no harm in asking.  Expect to pay $50-$100 for this level of training, plus the cost of ammunition.
All right – you’ve invested in a BB or Airsoft weapon and practiced enough to be able to hit what you’re aiming at.  That’s a great start.  Now, on your very limited budget, what’s worth buying for defensive use?
I recommend a shotgun as a basic home defense weapon.  It’s mechanically simple, usually pretty reliable, and powerful enough to stop most intruders in their tracks if the worst comes to the worst.  I’ve written about shotguns for home defense in a three part series of articles – follow each of those links to read more about the subject.  There’s also an excellent introduction to defensive shotguns here.  You can buy a perfectly serviceable new Chinese-made shotgun for $200-$250, and used models of US shotguns are available for similar prices.  Look for a shorter barrel (18″-20″) and, if possible, magazine capacity of at least 5 rounds – some offer up to 8 rounds.  If you want maximum ammunition versatility, go for the 12 gaugeofferings;  20 gauge is almost as effective (and my recommended choice for those who are new to shotguns), but defensive ammunition (buckshot and slug) isn’t always as freely available.  (You can get reduced recoil buckshot and slug ammo for 12ga. shotguns that’s easier to control than full-house loads.)

I recommend any of the following shotguns (if used, have an expert check their condition before you buy them):

• Remington 870 (manufacturer’s page here);
• Mossberg 500 (manufacturer’s page here:  I have several of this model of shotgun);
• H&R Pardner Pump (a Chinese copy of a US design – a bit rough, but serviceable);
• Norinco 982 (Chinese copy of a Remington design, similar to the H&R Pardner Pump).

There are many others out there that may serve you well;  but in terms of quality and value for money, the four I’ve mentioned have worked for me.  YMMV.  You should be able to get a new or used example of any of the above, plus enough cheap birdshot ammunition to become familiar with it and some buckshot and/or slugs for home defense, for not more than $300 in all.
If you want a handgun instead, that can be a problem.  A quality name-brand handgun will usually cost $300-$500 used, and $500-$750 new.  There’s a plethora of designs and variations out there, and choosing the right one can be bewildering.  However, if price is the ultimate consideration there’s one handgun that’s relatively low-cost ($175-$250 retail, depending on model and caliber), but runs surprisingly well in practice.  That’s the Hi-Point range of pistols (manufacturer’s Web site here).

I don’t like them;  they feel over-large, clunky and ungainly in my hands, and give the outward impression of poor quality.  Nevertheless, many people who own them report that they’re reliable and accurate enough for defensive purposes.  Those that I’ve fired have lived up to that reputation.  Therefore, don’t let my prejudices put you off;  try them for yourself.  They’re certainly much more affordable than most of their competitors.  The company also makes a line of pistol-caliber carbines that have earned a reputation for reliability.  (I actually like their carbines much better than their handguns.)  I’d still rather save my money until I could afford something better, but that’s my opinion.  A bad guy will hate to get shot by any gun!
I don’t know a low-cost double-action revolver that’s of adequate quality to satisfy me.  If I’m going to have to spend $300-$400 to buy a Charter Arms or Rossimodel, I’d rather spend as much or a little more on a higher-quality used Smith & Wesson revolver.  (Get an expert to check it out before purchasing it, to make sure you aren’t buying someone else’s problem.)
(EDITED TO ADD:  Reader Sport Pilot has offered one alternative that seems intriguing – see the foot of this article for details.)
However, there’s one low-cost revolver for trail and camping use that can also serve for defensive purposes in a pinch.  It’s the Heritage Arms series of single-action revolvers, available in rimfire and centerfire calibers.  Their .22LR and .22WMRrevolvers are very affordable, many costing less than $200 retail – I own a couple myself, and like them.  (I agree that .22 rimfire, either LR or WMR, is marginal as a defensive round;  but it’s better than nothing).  Their centerfire models are more expensive, but still offer value for money compared to higher-grade competitors.

Most instructors (including myself) agree that a single-action revolver is slow to reload and not as fast to shoot as a double-action revolver, making it less than optimum for defensive use.  Nevertheless, I can’t help remembering that the Old West was settled with the aid of such revolvers, and their modern users like the members of the Single Action Shooting Society (SASS – also known as ‘cowboy action shooting‘) demonstrate remarkable speed and accuracy with them.  (Look on YouTube for videos of their prowess.)  A single-action revolver will serve you as well as you train yourself to use it.
Whatever you buy, make sure you also purchase enough practice ammunition to familiarize yourself with your weapon of choice, and also buy some premium defensive ammo to take care of “things that go bump in the night”.  In today’s market, ammo can be hard to come by, so don’t buy a gun that uses a cartridge you can’t find for love or money!
In my experience, you can buy a BB or Airsoft training weapon, plus enough gas cartridges and pellets to train yourself to an adequate level of performance, plus a defensive weapon (one of the shotguns or handguns mentioned above), plussufficient practice ammunition to become familiar with it, plus a small quantity of higher-quality defensive ammunition, for a total expenditure of under $500.  If you wish to ignore my advice to get a BB or Airsoft training weapon and/or attend an entry-level training course, you can knock $100-$200 off that total.  I don’t think you’ll be able to do much better than that.
What do you think, readers?  Any suggestions?
Peter
EDITED TO ADD:  In a comment, reader Sport Pilot tells us about the M5 12-gauge shotgun and the M200 and M206 .38 Special revolvers, all made by Armscor in the Philippines.

I have no personal experience of these firearms, but I found this review of the revolvers that seems positive.  I was interested to read that one can use grips made for the Colt Detective Special to replace those supplied from the factory – an important consideration to improve a revolver’s fit and feel in one’s hand.  I’m going to see about getting one to test in due course.  Thanks for the heads-up, Sport Pilot.  At a retail price that appears to be in the $220-$250 range (if one can find them) these might be a value-for-money proposition.
A couple of readers have suggested single-shot shotguns like these (a review is here).  I like them as ‘fun guns’, but I tend to argue against them for defensive use, because for not much more money you can get a pump-action shotgun with greater magazine capacity.  However, for those with so little money that they can’t afford any of the firearms I’ve discussed above, I suggest these as viable ‘last choice’ weapons.  I’ve found several used examples for disabled students who had less than $100 to spend.  They were able to buy the gun, a 25-round box of birdshot for familiarization and training, and a 5-round box of buckshot for defensive use within that budget.  It’s hard to beat those numbers.