Michelin actually own the track and had it built as a test facility in the early 70s. I've no idea how much it cost to hire the circuit but the cost to individual riders on track days is very reasonable. Since I have been going there it has grown outwards and now incorporates another separate race circuit and various off road tracks for testing all sorts of vehicles.
I just go up there and take "pot luck" but mostly it's corporate days for motorcycles and occasionally cars on the actual original tarmac circuit with a lot of "born again" bikers reliving their "yoof" ! The winter testing for the various championships can be "locked down" but I normally find a way in.
This is from their blurb.
In order to ensure customer satisfaction and safety, Michelin must evaluate the performance of its products on test tracks. Its Centre d’Expériences Michelin AlmerÃ*a (CEMA) experimentation center in Almeria, southern Spain occupies 4,500 hectares in a beautiful desert area with the lowest rainfall in Europe. Michelin decided to integrate preservation of this area as a core objective of its presence in the region, proving that technology and ecology can go hand-in-hand.