Welcome to the forums Andrew.
First tip is that search is your friend - there are 26 threads in Painting & Conversions with airbrush in the title, from just the past year.
As you'll see in many of them there are lots of variables when it comes to airbrushes beyond simply the price you're willing to pay, plus you have to factor in the air supply which will regularly cost more than the airbrush. Not directly related to the cost of the brush is what you hope/intend to spray, which can sometimes rule out certain models and go more towards others.
Einion