In reply to mloskot: If you don't already have one, I'd invest in a map which distinguishes toll roads from toll-free motorways. For example, the motorway going through the Massif Central from N - S is largely toll free, while the Autoroute de Soleil is very costly. The IGN map which covers the whole country on one sheet gives this info. Available from
http://www.themapshop.co.uk/misc/aboutus.htm (fantastic service, btw, with knowledgeable people on the end of the phone!).
I agree to some extent about the frustrations of RN roads (we call them Route Gnash and Snarl) where speed bumps are a total pain. But this is just the result of years of dangerous and inconsiderate driving by the French -something had to be done. My advice is to have a road atlas with enough detail to see how many settlements your proposed route has to pass through and base your decision on that. And don't forget many of the D (departemental) roads are very straight, direct, empty of traffic and less affected by town speed limits than the RNs. A bit like driving in the UK in the 1950s!
But ultimately, only you know how relatively important speed of journey and expenditure are.