Diesel is much cheaper in France. Petrol however is not.
Group drive is the most flexible option as you can take what you want without weight restrictions, and works out cheap if there's enough of you.
If you're taking your own bikes, take spares, lots of spares! Alpine bike shops aren't cheap and they may not have what you need anyway.
Another cost consideration is the amount of servicing that will be needed on your bike post trip. Especially if it rains out there. Be careful with any provided bike washes too, they may be high pressure. Either of those can have you servicing bearings all over the place (and in my case, forks needed doing as well).
However... it's well worth it.
Passportes du Soleil is a good event to get an introduction to the area, lifts open to bikes and then you could spend a week playing in the bike parks.
p.s. if you do drive the toll roads in France, get yourself a tag for the tolls so you can just drive through and avoid the hassle of cash/cards. There's a link to it on the Eurotunnel web site, I forget the name of it.