Cheapest price for LPG that I found in France (between 10th and 26th August 2012) was €0.68 at an Auchan supermarket just outside Tours. Most other places €0.72 - €0.84 on main roads, €0.88 - €0.97 on Motorways and Toll Roads.
Some of the supermarkets that sell petrol, shut and padlock the LPG pumps at 8:0pm as there is no facility to pay at the pump, like with petrol and diesel.
In Spain, prices range between €0.74 and €0.78 - and I noticed that a number of small tanks with a single pump have been recently installed on Repsol sites between Burgos and Irun. Still nothing between Burgos and Madrid, and Madrid and Granada though
Repsol seem to have the monopoly in Spain - I only found one non-Repsol station selling LPG, and that was a BP in Madrid.