I think £500/£600 is about right. I'm thinking back a few years to when you could do 150mph on the Autoroute and get a £30 fine and waved on your way!!
Also, don't forget, radar detectors in your car are illegal in France!! If they catch you with one, you are liable to a similar size fine and they will confiscate it (often stamping on it in the road in front of you
)