If you make the fee super high, then many email workflows completely break and sendgrid goes out of business.

I don't think there's a number where it does what you want and incentivizes sendgrid to be careful.

(And you might say to seek a middle ground, but I don't think there is one. My guess is that "too low for sendgrid to care much more about a couple percent of mail from hacked accounts" and "too high for sendgrid to still attract customers" probably overlap.)