[@Kho] there would have to be another factor adding to the 'cost'. At that rate, players would easily level up every turn and still have a tonne of Might spare, even without Sites.

Half again or twice your current level sounds more appropriate to me, personally. Haven't crunched numbers yet though. 

About worshippers: I am [i]really[/i] fond of BBeast's idea of Sites being effectively mortal spaces for divine benefit, that require mortals to maintain and/or capture. I personally don't think mortals should influence Might otherwise.