I agree it would be difficult to work out the details, but I don't see why using the previous year's tax return wouldn't work in most cases.
It will miss people who have lots of assets but no real income, but I don't know what could be done to counter that. Some people will always get away with stuff and fall through the cracks.
If your married, divide it by two. This should be fair enough.
If you lost your job and aren't making any money this year or feel the calculated amount is wrong, some sort of appeal process should be available. But if you're found trying to hide income/assets or game the system in some way, there'd need to be a penalty.
If you have no money whatsoever and cannot pay, there's always a jail sentence or community service available as an option.
I'm not sure what the 5th amendment would have to do with it. You're required to submit a tax return and report your income anyways. We're just fining someone a percentage based off that figure.
The 8th Amendment Excessive bail shall not be required, nor excessive fines imposed, nor cruel and unusual punishments inflicted
The fine would be a percentage of your income, designed to hurt your pocketbook proportionally the same based in your income, to act as the penalty for breaking laws. It is the same proportional fine to everyone. The fine for a speeding might be 2% for example. A 50% fine would clearly be excessive, but 2% seems fair.
If you make $10,000 a year, you pay $200. A weeks paycheck. If you make $50,000, you pay $1,000. A weeks paycheck. Bring in $5 million? That will be $10,000... A weeks paycheck.
This seems fair to me. Remember, it's not really about the dollar amount of money itself, it's the % penalty for breaking the law. The penalty of that traffic ticket should hurt the same no matter your wealth. Otherwise, those laws only effectively apply to the non-rich. If that's how it is to be, why not just re-write them to say "It is unlawful to exceed a speed of 75 mph, unless you make at least $400,000" or some other such nonsense?
Of course, nothing like this would ever work if the individuals or agency enforcing the laws benefited financially from collecting fines like they do now. Luxury cars would be getting pulled over left and right for everything and anything.
Anyway, it will never happen. People with money and power control most things, and they're unlikely to vote in something that hurts themselves.