Damn. There's not really an answer to that. Generally if it was at high revs, it would do more damage than when idling. But really the only way to see if there's any piston / valve damage is to take the engine apart and look. You may be lucky. But... and this is just a guess. To replace the cam belt and new tensioner you'd probably be looking at a couple of hundred. And for an engine strip down just to assess the damage you'd be looking at another couple of hundred just in labour. Depends on how much the car is worth.