What would be the cheapest way to settle this? Can I found a SaaS company that uses AGPL code, grow to ~100 usd/month and basically ask the FSF to sue me? Even if the legal fees alone amount to tens of thousand dollars, that should be low enough that a small company or a motivated (and financially secure) individual could try.
The first hurdle is to get it into federal court (since you probably want this tested beyond your current state). To do that, you'll need someone from out of state to sue you.
The next hurdle is how to get this case resolved in other federal courts. You could either setup the same scenario for each district (requiring that your 2 parties have businesses in 2 states in each of the districts) or you could try to appeal all the way to the supreme court. Tricky, because this really isn't likely to be a case that the SC would take up (it'd need the lower courts to disagree with each other. Further, they'll usually not do it unless the district courts also disagree).
Overall, the whole thing would be super expensive to pull off.