Of course, that's fair. But a good way to go about this is to ask a fan of the club how much they would pay for their best English player.
Ask a Chelsea fan how much they'd pay for a twenty year old Terry, or a twenty year old Lampard. Ask a RS how much they'd pay for a twenty year old Gerrard. Ask a United fan, or even yourself, how much you'd pay for Rooney if you could redo his transfer, knowing how good he's become.
I'd certainly say £50m would have been a fair price for Rooney, considering the player he is. Apply that principle to Stones. Yes, he's not the finished article, but if he keeps on, he's going to be the next Terry/Ferdinand and he's going to be vital to whatever club he is at (be it us or not us) for over ten years.