He's worth whatever a club is willing to pay, and unlike Stones, he wouldn't fit the criteria for a Guardiola vanity project.
I assume that's why he's biting the bullet and running down his contract. It's actually quite admirable, considering how many players sign bumper new deals and then kick off when their club asks for a prohibitive fee.
I think he signs a new Contract after the January window making him the highest paid Everton player with a release clause of 60-70 M. Would seem to make sense.