http://www.zagaz.com/autoroute.php?reset and enter the motorway (and direction) you're travelling (eg A26 Calais-Troyes, A1 Lille-Paris) and this will call up a list of "stations sur autoroutes" with the current fuel price at each.
You'll pay a lot less if you get it at a Leclerc or Carrefour hypermarket, it also tends to be cheaper in Brittany. (Intermarché aren't too bad either) .