I don't think I'd sell him for any amount of money beyond the absurd (I.E. enough to build a new ground).
The Bale sale shows that many good players are not better than one world class player. And while Ross isn't world class (or close to Bale's level) he has the potential to be. I don't think we could buy a player of Ross' potential with the money recouped from Ross + strengthen other areas of the team.
What Ross lets us do is focus on other areas of the pitch and look for good second options in his place.
Even beyond all that, he clearly will be better in 2-3 years than he is now - if we must sell him, sell him then, when he has a higher valuation.
If it doesn't work out, oh well, but letting go of an Evertonian wunderkid is not something we should be doing again.