typically, cost of repair depends on the shop, but you can either install new pistons with maybe forged ones which are stronger after the block has been honed(or machined depending on damage of the walls). or you can get a whole new sti shortblock from subaru for ~$1800, and then a gasket kit which is about $250, and then add labor for pulling motor, installing parts, and reinstalling motor.
-does your car have modifications? does it have a tune? or is it stock?