There's plenty of space in the middle though, no? Bank accounts cut you off if you hit a zero balance, or they can execute your transactions and charge you a fee. Why can't these services implement throttling or even halting if the charges hit a certain ceiling?
So you want to add an overhead for each query to check an internal LRU cache that then checks the billing system? Just the overhead of hashing the query into some cacheable identifier will hurt performance.
