Kent refers you to page 9 of Rick's France book which, for instance, says that you can get by with spending an average of $150 per person, per day for room and board, outside of Paris, and assuming 2 people traveling together. Of that $150, $70 is for lodging, with two people splitting a $140 hotel room. Throughout the book, you'll find double hotel rooms listed in that price range. For instance, I open to the Dordogne chapter and find many double rooms listed for well under €100/night. And, if you ever need to just pull over at a cheap, formulaic motel, Hotel F1 or Ibis Budget have lots of locations on the edges of towns, handy for drivers (see also p. 1051).
The eating part of the budget is $15 for breakfast, $20 for lunch and $45 for dinner with drinks; which is also doable, as long as you pay attention. Myself, I've often found attractive 2-3 course dinner menus around €35 without trying too hard. French hotel breakfasts are usually pretty simple and optional and are not included in Rick's hotel price listings, unless mentioned.
Your car rental will be a fixed price, planned in advance. Gas, parking, and road tolls will add to that. Page 1105 suggests about €15 for two hours drive on the autoroute. www.viamichelin.com is one site that helps you anticipate tolls as you plan your driving route.
Sightseeing costs are also addressed, recommended at an average of $35 per person, per day.