—“How does one find out if taxes are indeed limited? How does one figure out whether “the price is right” under a monopoly?”—
I think what you might mean is how we know what commission to calculate. And given that commissions and sales taxes are well understood phenomenon I’m not sure how that’s particularly difficult. Most shopping malls charge ‘commissions’ or ‘fees’ that are a percentage of revenue. They can be universal or particular, but that question is empirical not moral.
***As an adjunct to rule of law, and therefore free of discretion, the formula must remain constant. But the formula itself is irrelevant. The point being that the purpose of any producer of commons would be to increase common revenues, not decrease common profits.***
In other words, the problem is in providing the correct incentive to those who specialize in the production of commons, and to prevent loading framing and overloading in their arguments. Otherwise a sales tax that individuals vote in favor of initiatives is kind of hard to argue with.