Cost stops depending on how much the logic does

A predicate has a fixed size, so there is nothing to meter. Nobody estimates gas, nobody pays for a transaction that failed anyway, and nobody is surprised by a state-dependent execution cost. The fee for a given transaction is the same tomorrow as today, in units of the network's own asset.

That qualification is deliberate and it stays. Static in units is the claim this design supports today. What a fee converts to in external value moves with the asset, as it does on any chain, and whether the protocol can offer predictability in value terms is an open design question, taken up on the economics page. We say predictable rather than fixed.