Right price is very hard to define due to his potential.
He's great now, worth maybe 30m.
If he fufills his potential at Chelsea and helps them win five league titles and two champions leagues as captain. What's that worth?
If he stays with us and guides us back to the top tier, say we win the league with him as captain just once over the next ten years and regularly qualify for the champions league. What's that worth?
What if England win a trophy with him starring? What's that worth to the club he's at?
I'd say he's worth more to us than he is to them. They can sign a centre half of similar quality or near enough and have a chance of achieving their goals. I very much doubt we can. Even with 70-80m to spend, Liverpool and Tottenham have shown it probably won't work.
Got to keep him for as long as possible. Offer him a new contract next year that gives him 50% of the value of the transfer fee if he stays for three years or more. Takes money out the equation, he'll earn more staying with us than leaving and we get a chance at being great.