For various family reasons I keep track of Stade Brestois from a distance.
They are a bit of a yo-yo club, though they are doing very well this year.
They have had some fantastic talent in the past; Ginola, Ribery, a few others.
The French leagues aren't exactly competitive outside of the CL places, but Brestois have struggled to stay in Ligue 1 over the long term, mainly due to their finances. They're a selling club, you'd say. They have a deal and permission to build an incredible new stadium, which seems to have hit the buffers and I don't think any work has been started on it. If they could get it built, that would send them them into a different place financially, but for now they very much struggle.
Their club record signing was about £5m, which gives you an idea of their size; similar to most Championship clubs. One of the posters above mentioned that they must have a lot of great midfielders if they can let Dembele go -- and that is the case! They picked up a player from Norwich in the summer who is having a blinder, and they have a Del Castillo on the wing who is an absolute Rolls-Royce. They are a selling club though, they let one of their other good wingers whose name I forget move to Germany for £8m in the last window.
Short version: Dembele will be for sale for the right price, and I suspect that may be lower than we think. Whether he would want to go from French L1 to England League One is another matter.