Some countries have laws that make fines proportional to both the gravity of the offense and the offender's ability to pay. Finland is an example, unless they've changed their rules or I heard wrong about them in the first place.

Our system of assessing fines only on the basis of the gravity of the offense makes fines inconsequential as a deterrent to wealthy people, but I'm not sure US politics would have a place for progressive fines. After all, even progressive taxes have fallen out of favor; the Social Security taxes on minimum wage income are roughly equal to the total taxes on a million a year of investment income.