Currently, the rich can afford to break the law more so than Average Joe. If the fines are proportional to wealth / income, the footballer who speeds through a trap will have to pay a £1million fine, whereas a checkout clerk will pay £30. Both are hit just as hard.
Mainly for the traffic fine area, but can be used across the justice system, a persons fine for an offence should be related to their wealth / income. In that way, the affect of the fine will be the same for a minimum wage worker and a multi-millionaire for the same offence, even though the actual amount of the fine could vary substantially.