Make it a percentage of disposable income, calculated as income plus 4% of net worth minus average living expenses for your city.

To avoid letting low-income people commit certain crimes without any penalty, maybe have a minimum fine but allow anyone who would be eligible to pay less than the minimum to make up the difference with community service (i.e., if the minimum fine is $200 and the calculated percentage of their disposable income would only be $100, they can pay $100 and then work 10 hours of community service).