It is true that dealers receive a set fee for Motability orders (it’s not £500 – I can’t recall the exact figure but it’s less than half of that). However, Motability sales DO count towards manufacturer, franchise and dealership volume based bonuses, so the dealership can end up making thousands out of your Motability order and the set fee is therefore only part of the story.
I have a BMW X1 on order as a Motability car and managed to get around £800 discount (combination of discount off the AP and optional extras) and you can be sure that the dealership is still making money out of the transaction!