This question pops up again and again. What I find funny about the answer is that airbrushes are actually fairly inexpensive. That $20 one Jim is talking about probably would work quite well. Even the top end 100 (gravity fed) or 150 (siphon fed) dual action series from Bear Air or Dixie Art are fairly reasonable at under $65. What bites is the cost of the air supply. You will easily spend two, three or four times the money buying a compressor.
I agree with Jim, the cheap (emphasis on cheap) Testors model will probably do more to dissuade you from using an AB.
Airbrushes are nice, brilliant in fact, and they will definitely take your building to a new level, but you can also get impressive work with a brush. If all you can afford is that Testor, I would recommend sticking with brushes!
If you are set on getting one, and are on a limited budget, you really need to think about how you can afford one, maybe save up the money over a couple of months before you can buy the AB, then save up more for the air supply. That takes a long time, but if you really want something bad enough, time passes fairly quickly.
There are many hurdles to get over:
What kind of air supply can you use: Hobby compressor (fairly quiet and small). Garage compressor (large, loud and best used outside of the house, can you run a hose into your build area). CO2 air tank (fairly expensive to buy, fairly inexpensive to refill, absolutely dead quiet, can be a PITA to get refilled if you rely on public transportation), simple air tank (fairly inexpensive to buy, you can fill up for free at the local gas station, and you will run out of air at the worst possible time).
Where will you use the AB: basement, hobby room, kitchen table, garage. Are you in an apartment? A house? Do you share your space with others that will resent the noise and smell?
Then you need to think about getting a spray booth, something that can contain any overspray and evacuate the toxic particles away from you and yours. You might be able to get away with a cardboard box and a fan blowing away from you and towards an open window. Can you build one yourself, or will you have to buy one? A good spray booth can make the compressor look affordable! Respirators are recommended, that's an additonal expense many people neglect to remember.
Daunting, isn't it? Airbrushing isn't cheap, I've easily spent $250 setting up my system (Badger 150, garage compressor and 50' hose, custom home built spray booth with fans, ducting and doors), and I still suck at using the darn airbrush! But it's done and all I have to do now is learn how to use it better. An investment that is well worth the expense.
Good luck with your decision making!