I agree that everyone should be equal under the law, but that doesn't mean that fixed fines are fair. The same amount of money has a different value to different people, and that perceived value changes depending on one's income and wealth.
IDK if you saw my edit in my previous response with the community service example, but I think that might help clear up where we're diverging. If it takes me 10 hours of work to make enough money to pay the fine, but it takes you 100 hours of work to pay the fine for the exact same offense because our salaries are different, were we really punished equally?