Realisticly- looking at the Sterling deal, the number is around 35-40. Strikers bring a premium and the move is similar, big club to XL club. I don't like it and if I had my druthers the lad would play for Everton for...um, well... ever. It is the way football works though, it is a business built around sport, not sport that happens to make money.
All that said, in one year's time the deal is likely to be even higher if Stones continues to build and especially if he ends up on the pitch at euros next summer. So there is some calculated risk in waiting.