>Contracts are written to minimize costs already, and that's why they're long.

See OP. There seems to be this thing called "legalese" that makes contracts harder to understand than necessary.

In any case, if litigation risk is a major cost of a contract, then people drafting contracts will incorporate that factor alongside character length.

Overall, I think you may have a point. So my updated take is: Make the per-character tax low enough that it's not a factor for B2B contracts where it's standard for lawyers on both sides to review. However, make it high enough so that it's a factor in rental contracts, employment contracts, and EULAs, where at least one party typically doesn't retain a lawyer.