Depends on the condition....what airbrush it has, how many paints really. For instance, brand new models with an airbrush as shown in the first link, come in at about £250
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ttp://www.graphicsdirect.co.uk/products/63/Complete-TC2000-Airbrush-Kit.htm
Whilst you can pick up an secondhand older model on ebay for less than £50...then add the bits you want
sim air compresser tc 2000 on eBay, also, Air Tools Compressors, Garage Equipment Tools, Cars, Parts Vehicles (end time 10-Dec-07 14:07:48 GMT)
My honest rule of thumb when it comes to compressors and the like is, if you know where its coming from and can be sure of the condition, then secondhand is probably OK, and dependant on what your getting with it....£70 isn't bad (not a fantastic deal but not bad), but I personally wouldn't give myself the headache that could come with an old model that you have no history for.
Likewise, it depends on your requirements...if you are looking for something to play with, get used to, and see how you you get on before investing a lot of money in new, then its not a bad idea, but I wouldn't be buying something that was business critical that is secondhand and you are not assured of the condition.