Just at Coventry in the past few seasons we have had rave reviews for Turner, Wood, McPake, Cameron, Keogh, Cranie, Grandison, Dann, Ward .... even Barnett was decent as proved with Norwich but blighted by a couple of big errors and sending offs here. May even be some others I've missed too and yet players who came with a rep and pedigree, like ADZ were awful.
Ben Turner imo is excellent, I would be loathed to lose him, but as per that list above central defenders at this level are replaceable. They do not come up agaisnt anything like the class of forward that you get in the Premier League and for that reason I would always sell for £3m upwards. We are ok in that department, but solid replacements can be bought for less. Strikers who can score your way out of this league are priceless.
If we were building for the future and had genuine PL ambition, then we ould need to keep BT, because eh would cost a lot more to replace post-promotion, but as staying up is our realistic target this season, then we can afford to lose him, if it means that we can get a goalscorer in and can hold onto all our promising youth for longer.