In reply to Squashringer:
I've done this drive loads of times, solo, with friends and more recently with kids. Best option comes down to how you value time against money.
If you're in no hurry, cheapest but slowest option is to avoid the peage the whole way. Fuel in Luxembourg is often cheaper than France, so filling up there saves a few more £s. It's a pleasant drive, lots of small French towns so chance to stop at cafes and buy croissants from boulangeries. It's not really practical to do this route in one hit on your own, but with two drivers would be just about ok as long as you kept a careful eye on breaks and tiredness. But going back to the beginning, if you're in no hurry, break the journey up and enjoy seeing a bit more of France on your journey.
If time is important, definitely take the peage. It's quicker, but it's also a much less tiring type of driving so less time needed to recuperate before getting stuck into activities in Courmayeur. Google Maps will tell you the quickest route. Depending on ferry times and number of drivers you may want an overnight break, we usually find Dijon area a good bet.
Whichever option you take, I would definitely use the MB tunnel for the final part of the journey, anything else is just not worth the effort. They do a return ticket which saves lots of £s over 2 singles, but it's only valid for 7 days.