When our house was built, nearly 20 years ago, and we were having underfloor heating pipes put in, the floor was done in two stages. A rough concrete floor was put in on top of concrete beams over the foundations, leaving a vide sanitaire. Relatively thin pink plastic insulation panels (possibly expanded polystyrene) were then put down. I think that the pipes were installed in conjunction with the insulation, as I recall that some cutting of the panels was needed. (It's a geothermal pompe à chaleur system.) An anhydrite-based chape liquide was then pumped over the lot using special equipment. It's certainly worked well over the years but I can't see that the insulation panels can have been very expensive.
It's possible that the panels you are being quoted are polyurethane, which I would expect to be expensive, but are maybe needed to meet the French RT2012 requirements. OTOH, if they cut your fuel bills significantly, it may be a good investment. Having been down in my vide sanitaire recently, for the first time ever, I was struck by the possibility of adding more insulation down there by blowing in a sticky foam insulation so that it adheres to the underside of the beams. It would clearly have been more efficient, and cheaper, to have installed the type of sandwich insulation that you mentioned. In the coldest of winter weather we can occasionally get a certain amount of ice forming in the drains down there, so it must certainly get pretty cold, making for a large temperature difference and subsequent heat loss.