Is there a "sweet spot" for a player's selling price?

Recently Real Madrid tried to buy one of my prospect

http://cloud.steampowered.com/ugc/524907827303043309/4992298A94A33A29EAC6E14463F7AD1BB6975FA7/

they offered 2.5m with a 30% future selling profit. His potential is rated 2.5 by my coaches/scouts and for the reference players like aaron ramsey/romelu lukaku is rated as a 2.5, and yes it's only potential but I'm sure I can max his potential given time so I was keen to develop him but I won't mind selling him given the price is right.

In my experience hot prospects usually costs about 3x their face value and in the past hot prospects from other clubs that had been sold usually did got sold for about 3x their value. So I negotiate with a 7.5m offer but on top of that I also asked for 10m more after 40 league games . I figured if he can make it 40 league games in RM that means he'll be good enough to hold down a first team spot, if not then chances are he won't ever reach 40 games and got sold before the condition got triggered... obviously the A.I. disagree and admittedly that's probably asking a wee bit much .

But is there a good price for selling a player that you think might become a good player but don't want to sell him to a rival for a bargain price? Should the 3x base price be more than enough for it?