Fractional fines are perhaps better than flat-fee fines for law breaking without a clearly assignable dollar value, but still face the problem of log-ish utility of money. A person living paycheck to paycheck will be extremely burdened by any kind of fine, whereas a richer individual is much less likely to be, and even if they are - have more ability to make downward lifestyle adjustments to absorb the burden of the fine.

To put it another way, it is more painful to lose $250 when you're making $25,000, versus losing $3,000 when you're making $300,000.