We are talking about a young centre back who scores goals. English, so no problems with EUFA rules, which means lots of clubs will be sniffing around. Don't undervalue him and I am sure TB doesn't. We do not have to sell players except on our terms, remember that!
Young, English and scores goals. I would not be happy with less than 5 million and only then if it was to a PL club to advance his career. Spurs would be good for him, possibly Arsenal as well.
Really I would prefer him to stay and reach the PL with us. It's the hope that kills you!