the price of an airbrush varies, depending on the quality you'd like.
There are 2 main types, a single action airbrush, which basically has an on/off trigger; no direct control over the airflow or how much paint you spray, besides a knob for the paint flow.
The second, more advanced type is a dual action, or double action, airbrush, which allows you to control the airflow, and the paint flow with the trigger alone. This type is more expensive, but allows for more detailed airbrushing.
The sad thing is, the airbrush is not all you need. You have to have either a dedicated air compressor, or canned air with a $12 adapter. I wouldn't recommend canned air, as the pressure drops rapidly, and the cans are quite expensive for how short the last.
All in all, you're lookin at around $200-$300 if you want to get the airbrush and a decent compressor to power it (normal air compressors work fine, but may be too strong).
Hope this helps