Prices vary, but if the bike is used, I'd definitely take it for a spin and check for things like how smoothly it shifts, turns etc. Also, if the previous owner liked to do wheelies, check out the forks for evidence of leaky seals (from coming down too hard), glance at the rotors, brake pads, sprockets (for the "chainsaw" effect"). Check the chain tightness, if it's loose or rusty, little things like that reflects on how well someone takes care of their shiznit (ie If he neglects the chain, then what else?) Check the oil, see if it's really dark or low, maybe he didn't change it often enough, or likes to run the bike low on oil. Look underneath for signs of scraping or whatever.
Just look the whole thing over and don't rush yourself on buying a bike.
Oh yeah, check to see if he has an extended warranty (Y.E.S), because it's transferable.
Good luck.