Ruger P-series guns don't sell for much new anymore. but like Haranguer said, the ammo and magazines push this one well into the great deal range. They have a reputation for being extremely tough, most people find the P90 to be extremely accurate for a service-type pistol of any cost, and getting spare parts for it if something ever breaks shouldn't be a problem, Ruger isn't going anywhere and they made lots of those pistols.
I'd say just look for any major obvious defects, but with a gun like these ones if it looks fine it almost certainly is.