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
In reply to blurty: A midi ticket is 50 Euro return. You can get an unlimited for 53 euro a day (+plus an extra 3 euro if you don't have a card. An unlimited get's you to the top of the Midi as well as the top gondala for GM and also covers the Montenvers Train and Mer de Glace and Les Houches ski area. You can get a 5 day Chamonix pass non consecutive but this works out quite a bot more expensive (around the same cost as 6 days) you dont really get much of a discount for bulk day buying. Best bet and what I normally do is get 2/3 days Cham le pass 1/2 days Unlimited then 1/2 days Cham le pass. As far as I'm aware you cannot upgrade, you will just pay full price for an unlimited day ticket