If your goal is deterrence you have to ask who's more likely to commit an offense. When you lump someone into a group and apply different levels of punishment based on their group, that's just wrong and evil. You think its fine because [group] is some group you may not like (rich people). But if you start treating people differently based on [group] you risk losing control of who is defining the group. The simplest and fairest way is to treat everyone equal in the eyes of the law. That's the ideal and when possible we should reach for that ideal

When the consequences for the same crime can vary from [entire paycheck] to [below rounding error on personal finances], the effect of treatment is not equal.

When people get different punishment for the same crime based on their group, then the effect of treatment is not equal

Everyone can pay the same percentage so it is equal.