In fact there cannot be equilibrium rate of % at all because storefront's expenses don't scale linearly with combined sales. For example, storing binaries and other auxiliary data can have particular cost per megabyte, but game's storefront cost is entirely different thing. Some cheap/free game can weigh as much as some expensive one nowadays. There are some other expense factors too but you can be pretty sure they don't scale linearly with storefront cost. Some games will have to subsidize others in practice and due to many factors involved exact relation of income/expense can vary wildly for particular game, and this would be changing with time too. So they'd have to take lot extra dough in practice to make a buffer for potential shortfalls.