When a player moves to England, there is a fairly complicated formula used to establish the development fee owed by the club that signs him. It's based on a few things, including how many years he spent at the parent club and what division the new club play in.
The longer the player has been at the parent club and the higher the division the signing club play in, the bigger the development fee.
So it's an educated guess on how much we can get for him, but we have Lee Erwin as a benchmark, so it's a pretty reasonable guess.