viamichelin is the site to use for tolls as has been suggested. There is a site for all the autoroutes, but it's easier to get michelin to do the work for you.
One other point - fuel is more expensive on the autoroutes - recently i found diffrerences of over 10 cents/litre relative to the supermarkets, so you may want to refuel near your stop off points rather than on the toll road.
From Roscoff it's not too bad, as you can get just to the south of Nantes without using toll roads - still on 4 lane - and then you can cut across to La Rochelle - the old road number is N173 and it is still possible to follow it all the way down to Bordeaux. The autoroute south of Bordeaux is toll free down to the Dax area.