you can also make it have a certain amount of essence to get x2 (non permanent)
I have something similar planned for stat training locations. Gold in the game is kind of in a weird spot right now where it's almost worthless, so I'm going to be adding an permanent upgrade system to the stat training spots with gold, one of those. One of these new upgrades could be that you can spend a certain amount essence and increase the training amount by a proportional amount, for example you could spend 15 essence to increase the stat exp by 150%.