No idea how legit it is, but read an article in the Birmingham media (from just after we signed him) and they said it's 10% on the profit.
Assuming that's true, then Brum would get nothing on a £15m sale.
This (from tuesday) is still a really good post. You could argue more than 7 if you factor in a Perraud / Tino replacement, too.
I think, after last week, the squad is almost at a point where you can start to directly replace people leaving.