It's a good question, but for multiturn conversations even cached context adds up quickly. My experience has been that spawning off subagents for defined tasks in a large overall plan generally makes me come out ahead.
I asked this question and was told that even if it is counter intuitive, medium will be more cost efficient due to caching. Changed to medium, blew my budget and went back to low.
I'm sure folks' mileage will vary though.