A $50 fine will punish someone with $20 in their bank account a lot more than someone with $20 million.
Lets make a imaginary extrapolation to see it in other light, lets say that there where a second human like species here on earth, but they live until around 1000 years instead of 100 but they experience time 10 times quicker, so for them 10 years feel like 1 year to us. Would it be fair to do the same time in prison for them than us for the same crime? what feels psychologically like 10 years and 10% of our lives would feel to them like 1 year psychologically and will be only 1% of their lives, and if we started to put harsher penalties to account for them, they still will end up with kinda light sentences and normal humans would end up with absurdly high sentences.
If the idea is to punish so it is not done again, sentences have to be proportional to the one receiving the sentence to be effective if not you are over-punishing some are under-punishing others.
OFC this is with the idea that fines are punishments and not just paying for what you broke kind of deal, for example if it is a fine for parking in a place with a parking meter without paying, the fine should be what you did not pay plus a little bit extra.
You're not punishing them more, you're punishing them similarly: the marginal value of money diminishes as you have more of it, a $20 fine to someone who earns $5000 a week is a lot less punishing than to someone who earns $500. By scaling fines to income / wealth, you're preserving the fact that this is supposed to be a punishment deterring from the act.
> so, perhaps a system of increasingly punishing repeat offenders maybe?
That's... already common? But if you have enough money, it would have to scale up quickly and drastically to have any deterrent effect, and it would still disproportionally affect lower-income individuals.
An alternative (or complement) is to create a flat resource, that's what points systems do (although they tend to still disproportionally affect lower-income individual: they're harder hit by the loss of mobility, and they may not have the time or money to attend to recovery courses when those are available).
Any fair punishment will disproportionally impact lower-income individuals, since resources confer resilience and resilience resists punishment. If your goal is equal impact, you have to have unfair punishments, because you have to scale your punishment to the target’s resources.
I'm not swimming in money but I earn well above average, and because I do my disposable income is a higher multiple of the average persons disposable income than my income is. To be fined even, say, a few days income for me is at most a minor nuisance. Maybe I'll feel bad enough to eat out at restaurants less that month. Maybe. To be fined 1 days income for someone low paid might mean they'll struggle to cover their bills or eat.
There's nothing equal about even a fixed proportion of income for that reasn. But it's significantly better than a fixed amount that doesn't take income into account at all.
If you truly want to punish people financially the same in the sense of making it hurt the same for the same transgression, you'd have to fine people earning above average not just a higher amount, but a significantly higher percentage of their earnings and wealth.
EDIT: I'm reminded of Marx discussion in Critique of the Gotha Program of the first proposed program for what became the German SPD of the use of the term "equal rights" which, after tearing the term aparts ends with "To avoid all these defects, right, instead of being equal, would have to be unequal." That is, if you want equal outcomes, you can't treat people the same, because their situations are not the same.
People should just stop thinking about global policies, because it's a road to hell.
For one, this would mean if you gave someone a 50% discount on their parking fine because they were poor, but it turned out they actually only had 25% of the assets of the rich person, you are still unfairly punishing the poor person more than the rich person. It can’t be correct that an observer has to know the financial status of every citizen to judge whether their government is being fair or not.