I agre with the first sentence, but come on.....as poor as he has been and though I never rated him above a decent defender when we were linked....do you genuinely believe in the premiership market we would only get £15-20m for him, Michael Keane got sold for nearly £30m after all. No we wont get near £80m for him, but we would certainly get a lot more than you suggest, probably double. A "normal functioning business" certainly doesnt sell assests like that.
Forgettign the finances, the main problem you inadvertently hit on the head fo rme though "we dont have any decent centre halves" bar Varane. There are others that should go before him, irrelevant of his performances, I would also suggest a couple of them would raise the kind of fees you are suggesting for him too.
What we should be looking to do for me, is clear out some of those and sign a quality young centre back that puts genuine pressure on his place and future at the club, where he simply doesnt get in the side with his form and performances at the moment, then we can look to sell him further down the line. I feel similar thougth with Bissaka who I actually do rate though he hasnt progressed as required this season, he needs prssure, selling him an dkeepig Dalot and WIlliams wouldnt make sense to me, as both are worse and could raise reasonable revenue for a quality replacement to put pressure on Bissaka