The expected benefit is 25 cents if the minimum possible contents of an envelope is 1 dollar. More generally, if envelopes are guaranteed to contain more than 100 dollars, then using f(x)=100/x yields expected benefit of 25 dollars, and so on. Also, Eliezer is right that a randomized algorithm can't be Bayesian-optimal in this problem, so for any given prior there's a deterministic algorithm with even higher expected benefit, I guess you can work it out.