Look at Auto Europe for prices and terms. I've rented through them but you could try the individual companies websites to look for a better deal. There are rental locations around the Porte Maillot on the Peripherique at the west end of Paris, or maybe the La Defense area, which would spare you the worst Paris traffic, or take a train to Rouen or Caen and rent from there. As Ed points out (where is he anyway?), this may mean paying both train fare and car rental in the same day.
Gas and tolls are much more than in the US, but the cars get much better mileage, especially diesels with manual shift. They give you flexibility for areas like Normandy/MSM that trains won't provide. You don't need to return them to the same place as long as you bring them back to the same country, as you plan to.