For any reasonable email fee, sendgrid can continue passing it on to the customers and not care.
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.)
