As long as you're not bound on parallelism or bandwidth then it's "free", but if you're constrained on either resource then your lighter predictor model just needs to save you more cycles than it congests on average.

A bad guess still costs cycles, but the penalty is smaller compared to branch mispredict in the current state. But if we have some kind of pipelining, like if we have something that assumed the speculative decode is correct, then it'll be expensive again.