Difference being that Ferdinand was already a top, top quality CB when the rags bought him. Stones is at best a work in progress, especially at actual defending. He's far more comparable to when Leeds bought Ferdinand for £18m of West Ham, although I'm not convinced he's even at that level yet.Ferdinand made around 34m back in 02.
Transfer inflation would make the current price around double that paid back then.
The squad composition rules make English players even more valuable now particularly to teams like City who have few of them at the required level at the moment. His age also adds greatly to his value.
I would be surprised if Stones went for less than 60m unless he made a nuisance of himself at Everton.
Unlikely I know.