If you're gauge of what's a reasonable price is based on what you would pay in the U.S., you won't be very pleased here in France. Not only is the dollar at near-record lows against the Euro, but equipment is generally priced considerably higher than what you see in the U.S. The only gear I've purchased in the last year I've been living in Paris has been at camera fairs where I can bargain.
That said, I do know of a couple of shops on Beaumarchais that have used darkroom gear for sale -- if you're walking north from Bastile metro, they'll be on the left side of the street.
I do agree that Prophot has a pretty good selection of film and such. But oddly, they don't display their chemicals. I was dumbstruck when the sales lady said she had to go check their stock, since they "couldn't" put it out on display. A French photographer watching all this remarked to me that "this is
France, we can't just have things sitting out like that." I guess they're afraid someone will drink Rodinal and they'll be liable. Very odd.