it all depends on what sort of airbrushing your wanting to do weather its free hand or stencil work. for stencil work your looking at a single action air brush and any shitty cheap one from ebay will do don't pay any more than 15 quid for one.
if your looking to do free hand work then you will need a double action air brush where you press down for air and pull back for paint this gives you a fine stroke like a pencil to a wide stroke like a paint brush.
also depends on what paints you will be using eg. acrylic, solvents, water
based.
i have a sata double action airbrush and a devilbiss single action air brush and both cost me a little over 500 including a in line water and air filter and a pressure regulator.
if you have never done it before i would advise you just to get a cheap set up from ebay and have a play.