Personally I find this to be a horrible thing. Yes, I can appreciate modders wanting to get something for their work but don't make it a requirement, otherwise you'll likely drive off people that simply can't pay for mods in addition to paying for games. Speaking just for myself, I would avoid any mod that requires payment like the plague for the exact reason I stated above. I have used a ton of mods for Skyrim and for several other games but that was because they were free and added life to the game I had already paid for. I simply don't have the budget to buy both games and mods for them and when it comes to a choice between Game or Mod...the obvious choice is Game every time. By all means, if modders want to get some money from their work, make a system for letting people donate if they have the money to do so but don't lock those who can't pay out of being able to use mods, in the long term that will only kill the life support system that keeps games from dying after a few years.