There are no laws about this. Most publications tell you how much they will pay for reprints relieving you of the decision.
The reason most pay less for reprints is that readers many have seen it before. My thoughts are, if it's worth putting before readers, it's worth full price. Often that isn't much even at full price. If you approach a different market from those where it has appeared, I see no reason to lower your price. Charge what the market will bear.
I have actually been paid more for a reprint than I received from the first publisher of an article because of differing standard rates. In case you wonder, yes, they both knew about the other appearance.
celestewrites