There is some variation wrt prices in Western Europe with Denmark being an extreme high (LPG is taxed the same as petrol) and Belgium and Luxembourg (LPG having no fuel duty at all). I've found this site to be very useful: http://www.mylpg.eu/
but note that there is often huge variations in local prices compared to published averages (eg Netherlands average official price EUR 0.815, I filled up for EUR 0.64).
Worth noting is that Belgian prices have an upper maximum set by government so you're not taken for a ride if filling up on the motorway, but you can find even cheaper LPG off the highway. Last week I filled up for EUR 0.521 (that's 43.6p) per litre just 200 yards off the motorway outside Gent (Lukoil, Kortrijksesteenweg 1054, 9051 Sint-Denijs-Westrem) . If travelling through Belgium this http://brandstofprijzen.vroom.be/route.php
handy dandy route planner will show you the 10 cheapest stations close to your route, and in France this http://www.prix-carburants.gouv.fr/itineraire/
government-operated route planner will do something similar, very comprehensively.
In the Netherlands the cheapest I've generally found is Makro at EUR 0.64 or so per litre http://www.makro.nl/public/home/Vestigingen/Tanken
but only a few of them have LPG (you don't need a Makro card to fill up). I've seen EUR 0.66 on the motorway, which is very good; there is quite large price variation between stations even very close to each other with no obvious reason for a particularly high or low price.
If you're not afraid of a little linguistic experimentation, Google for:
For some reason "GPL bon marche" doesn't yield much for France.