Several countries use a dayfine system, we in Sweden have used dayfines since 1931, Finland since 1921, Germany since 1969, There are a few more countries using the system, but I could not quickly find any historical data about them.
A day-fine, day fine, unit fine or structured fine is a unit of payment for a legal fine which is based on the offender's daily personal income. It is intended as a punishment financially equivalent to incarceration for one day without salary, scaled to equal impacts on both high- and low-income offenders. An analogy may be drawn with income tax, which is also proportional to income, or even levied at higher rates for higher incomes.
Jurisdictions employing the day-fine include Denmark (Danish: dagbøde), Estonia (Estonian: päevamäär), Finland (Finnish: päiväsakko), France (French: Jour-amende), Germany (German: Tagessatz), Sweden (Swedish: dagsbot), Switzerland, and Macao.
A percentage of income still isn't equitable though.
If you're destitute a week's income means you starve.
If you're a millionnaire a week's income stings bit doesn't affect much.
If you're a billionnaire there is a good chance you don't technically have an income, and if you do you can lose half of your wealth without feeling it.
This is true, but you could still have a progressive fine. Very good point with the billionaire, though. They live in a completely different world, in terms of how their wealth flow works. Still, it seems like an alternative fine system could be worked out that would hit them hard.
The billionaire might not feel it, but the money gained could be significant for all sorts of good things that help lift the burdon of the lower class.
Would be nice to see fines replaced with community service in many cases. Though I feel like you would then need to ensure that those doing community service are compensated the equivalent of their wage(s) prior to conviction if community service requires you use hours otherwise used by your job. Otherwise, somebody dependent on their job effectively pays more for smaller fines due to loss of work. It would also help to prefer out-of-hours community service (weekends for example) to avoid losing hours from your job in the first place. Ideally, jail would be reserved for cases where the person is a genuine danger, rather than use it as punishment.
San Francisco lets people pay parking fines with community service, and I believe their answer to that would be they require sign offs from preapproved organizations. (Sure, somebody somewhere could take a bribe.)
Punitive damages shouldn't have a cap, and should be even greater than proportional, since it's easier for someone who is richer to live on a smaller proportion of their income.