The problem is it really isn't. We will be able to get 30m in another season's time and since he's got another 4 years on his contract there's no financial reason to sell him at the same price we will be able to get for him in 2 years.Stones is good but not worth £40 million. £30 million should be more than enough.
Mourinho has to pay a premium given the length of his contract and he knows it which is why he's trying to unsettle the player.
You might think that 30m is a lot to pay for a 21 year old defender who is "potential" and therefore a risk but the Everton board, manager and fans don't see it like that. To them there's no chance Stones won't go on to be one of the very best.