I suppose there's nothing requiring the proportion-curve for fines to be linear - but that might get silly and have other consequences. I guess the question comes down to value - finding something that the wrongdoer values (and remove it) that is in proportion to the damage to society they did. I would guess, that for the rich that we're talking about, time and/or freedom are more important than money.

The other end of the scale is interesting too - how do you disincentivise/punish for people who don't have any money and just aren't going to pay any fines you give them (i.e. the homeless)?