Tony Bloom has always said that he won't stand in a player's way if a bidding club meets his valuation. BDB's valuation may seem low, but we have to presume that it is impacted by his age, the time left on his contract and what the club think it would cost to replace him in the squad. Left footed centre back Haydon Roberts has been on the bench all season without playing. He has also played left back. I suspect that he won't now go out on loan. The club has Van Hecke and Clarke on loan in the Championship and two age range Belgian International centre backs in the U23s. I would imagine that this bigger picture stuff would have contributed to the club's valuation of Burn. I would also guess that the fact that contract negotiations are imminent for other, more financially valuable, squad members may also impact things. Selling Dan now evidences to others that the club will not stop squad members from moving if valuations are met.
The business model is about developing players and trying to sell them on when they reach their peak value. The club has obviously decided that C£13m is BDB's peak value. That value may seem low in isolation, but if the club feels that, for instance, giving 19 year old Roberts more opportunity to prove himself as potentially of greater value and more reason to sign another contract (his current deal ends the same time as Dan's), that could be reducing the level at which the club will deal for Dan Burn.
When we sell Roberts (or A NOther) to Man U for £60m in 2 years time (he was thrust into the PL limelight after we sold Burn you see, but did brilliantly), it’ll look like even better business