I don't quite understand the focus on the token cost of this rewrite. Obviously, we should examine if this rewrite is good, effective, good for the product etc., but the token cost seems ... not important?

The marketing value of this to Anthropic (if Anthropic even cares, this might just be the Bun team selling past the close) is to show that such a rewrite is possible and delivers engineering value. If exactly this project is $800K today, it'll be $200K and then $80K soon, so it's not so important to the story that it's cheap, just that a big "cool" rewrite is possible, and delivers velocity to the buisness.