It depends on what you want to use it for Chuggy? If it's for actually spraying models, you need a pretty good one (which costs of course). If you just want to do the odd bit of weathering, a cheapish one as often seen on ebay is fine. Then (like me) you might get into it and invest in a compressor etc.
A few years ago I visited a railway (US outline) that had some absolutely brilliantly weathered stock. The chap who did the weathering used to work as a proffesional modeller in the movie industry (including Starwars) and he reckoned that he never spent more than 20 quid on an airbrush. That was a while ago, so perhaps you need to add a bit to that, but I've still never seen better results.
