While I generally agree that Stoneskin is a bit too expensive for what it does, especially for a Sorcerer (where giving up a spell known is an expense), I'd have to wonder about this. Unless the Sorcerer only casts Stoneskin on himself, it'd seem rather stupid to make him pay for every casting of it. If I'm the group's fighter, or, perhaps even more likely, the group's melee-oriented rogue, I'd be forking the 250 GP over for any significant fight, rather than say, "Cast Stoneskin on me - don't worry, you'll still make a bit of a profit in treasure!" That, or make it a group expense - if the Sorcerer casts one Stoneskin per encounter, then the group gets a net of 2,350 gold, or 587.5 GP/person (compared to the 2,600 and 650 above0. Just the way I prefer to handle it, but I think it really lowers the perceived cost of Stoneskin