Ask the builder to make an offer for the land and see what he comes up with. Your mother wouldn't be getting ripped off, no matter what price she accepts, if she's happy with what she gets. The agreement should also be that the purchaser pays all the legal fees etc.
You could also look on auction sites such as Allsop's, who sometimes sell land as well as property. These can be very small parcels of land not huge areas. You need to register, for free, on some of these sites but you'll get to see how much small areas of land went for. Might give you an idea of how much to expect.