I work for a provider in France, and I'm not surprised you've had no luck in finding ad rates. It's something that's just starting up here. We tried to start it up earlier, but people just weren't buying it.
Prices vary of course.
I would say between 3,000-5,000 FF per month for a spot on a main page. Special deals can always be worked. Imaginet is starting up a program that will manage ad sales on the net. We offer publicity spots throughout our web, most particularly on the Virtual Champs Elysees. Don't lose heart, its picking up here.