All depends on the model. My 1.67 G4 still runs great and even runs cs3 at a respectable speed considering its a 3 year old machine. 350 sounds reasonable but it all depends on the condition, model, etc.
Personally, I'd pass without hesitation - it has a small hard drive and not much RAM so you're going to be limited to older versions of apps... but the price doesn't seem too far off - a quick Google for 12" Powerbooks (1.5 GHz) shows about $700, but that's surprisingly high to me. Good luck!