Suggest you look at the website for the airport (Bordeaux Merignac) and see what rental agencies are on site. Alternatively you could look up the UK car hire brokers Holiday Autos (
INVALID URL and see what they can offer in terms of price and vehicle capacity.
Normally the 'major players' Hertz, Avis and Europcar cover airports in France even if they have to bring the car from their city locations.
I think that the price you have been quoted for 10 days is quite reasonable as this is peak season and this part of the world is full to bursting with holidaymakers - often Parisians!!!
You might also decide to check out your journeys with
http://www.viamichelin.fr and 'bison fute' which offers alternatives to roads that are usually jammed solid during the holidays. The Bordeaux ring road, sadly has a bad reputation for getting snarled up, and also the road that links it to Arcachon.
Oh and if you haven't already booked accommodation, be it at the seaside or inland in the Dordogne/Bergerac area or even further south in the Landes, make sure you start early as capacity is limited!