In reply to blurty: If you're going for more than 4 days (I think that's what I worked out, could be slightly off though), you're better off getting a Rapidcard - basically a oyster card for lifts in Cham. You get charged a flat rate of 40 Euro per day in winter (which comes straight out of your bank account), whichever lifts you use in the valley and every 7th day is free.
Spring skiing and summer are cheaper still and there are some promo days that get advertised every now and then at cheaper rates. It lasts for a year and has the added bonus of meaning you dont have to commit to a 7 consecutive day pass but neither do you have to queue for ages for a ticket on a powder day - just join the lift line.
http://www.compagniedumontblanc.co.uk/en/lift-passes/chamonix-rapidcard
Cheers,
Rob