My main point was not about the implementation stage, but the verification stage. Letting the smarter agent verify the work of the cheap agent finds a lot of little oversights and sloppiness that I used to have to find myself. So I’d be interested in that cost comparison.

I think it would depend on how you measure it. Adding a review step will definitely increase cost, but theoretically that cost is lower than re-doing the subagent's implementation later when you discover it's broken.