Seeing the rough data there:
http://www.meilleursagents.com/prix-immobilier/
You could say that generally speaking, anything in France outside Central Paris and its western inner suburbs seems to be a fair deal, but of course things get much more complicated when you look at it in details.
The Mediterranean coast, especially the east side of it called Riviera, that comprises Nice and Monaco for instance, and Corsica can be overpriced too.
So as far as I'm concerned, I find Bordeaux, Lyon and Nantes are the current best deals in there. Not necessarily the cheapest, but development is fairly steady over these 3, especially in Bordeaux whose mayor might be the next president over here, and in Nantes that's doing much better than the rest of the country as a whole.
Many in Paris like Toulouse too, and would willingly move over there if they got an opportunity, but I think it sprawls a lot. Don't think they like urban density so much over there. They like their single-family homes with their pools in their yards when they can afford something comfortable.
Marseille and Lille might go up on everyone's list sometime soon, cause some interesting developments are starting to be implemented out there as well.
Even though I'm a local, I wouldn't particularly recommend Greater Paris for now. It's obviously and by far the largest, most famous, most urban and cosmopolitan, but often tends to be a little exhausting when you're no millionaire or when you ain't used to it enough. Dense cheaper areas are often rough here. And France is usually "nicer" anywhere else, especially at foreigners.