It bugs me that it is a cash payout. It SHOULD be hammers towards an equivalent non wonder building. Fail to build the great library? You should get hammers toward a library. Fail to build the great lighthouse or the handing garden? Hammers towards a lighthouse out garden. I think you could match almost every wonder to a non wonderful equivalent. Not a1 to 1 hammer conversion, but proportional. Built half the great library? You are half done a regular library instead.