The US Constitution really doesn't apply. These kind of violations fall under the police powers reserved to the States. And, in any event, a given percentage applied to everyone equally is not a "sliding scale, it is as "equal" as it gets. 

As for jury nullification what stops it now? Also with claiming hardship or destitution? Although, everyone has tax and/or welfare records available, and falsifying those would take a few hundred dollar fine into felony fraud and perjury territory. Many people act stupidly, but that's a lot of effort and a lot of risk to avoid what is still a small fine.

As for three strikes, forfeiture, etc. Again, this doesn't change anything but -how the fine itself is determined-, it has zero effect on any other part of the current process. So, whatever you get -now- for multiple offenses, or when you don't pay, you still get.