[Evilhomer]

As velusion said clams are very potent in the long games no doubt about it - especially after wish is reached. Removing them (or having some ridicilous spell being able to destroy them) is not a good choice however since you will basically cripple high astral dependent nations like R'lyeh.

At the current cost they will take 15+ turns to pay off basically, which is a risky investment.

I think a good solution is to mod them for whatever game you are expecting to play. In a short-mid game keep the original cost. In long games on large maps that is expected to go 100+ turns increase their cost slightly (by 5 water or nature gems). Of course in such games blood stones should have their cost increased as well.