Lost Mines of Phandelver mining share

At the end of Lost Mines of Phandelver, Gundren Rockseeker offers the players 10% of mine profits if they locate and clear the mine. My players chose to continue their existing characters into a new adventure with the expectation that they received their 10%. I don't plan to give my players an unlimited supply of gold from a mithril mine indefinitely. So, I generated this calculator that I could allow them to affect with d20 rolls, but that would eventually phase out their mine profits. In universe, this will result in the characters having a falling out with Gundren Rockseeker. They can choose to explore that or take this earning and move on.