Fines in Germany usually follow the "Tagessatz", which is the income of a day, as calculated based on your tax records (and other information). So you might be sentenced to pay 90 'daily incomes', which hits a poor person as badly as it does hit a rich person.

Also note that there is a minimum amount of income that you always will be granted access to ('Pfändungsfreigrenze'), so a fine will not make you die from hunger nor make you homeless.

True in principle, but afaik the income is usually estimated (it is not measured precisely) a lot lower than its true value for wealthy persons. Still a better idea better than fixed fine amounts, of course.

people scraping on their salary are hit very hard by fines defined by "Tagessätze". But once someone is "poor enough" to hit the limit or anyway living on welfare they are very ineffective.

Prison is far worse for the poor than it is for the rich. And the poor are far more likely to end up in prison over the most ridiculous crap in the US. In practice, the US's justice system is far worse for everybody but the rich.

fines in germany are completely ineffective against criminals on welfare. (and that's a problem)

That's not true for criminal offenses at least. You have to pay them or you go to jail. If you're on welfare and have 400€ to live on they generally expect you to pay 100€ a month. From what remains you have to pay everything but rent, so energy, Internet, and other fixed costs you might have as well as food. I wouldn't be surprised if this actuajly forced a lot of people into crime just to not go hungry.