There’s still plenty of riding to be had in Chamonix once the July – August ban kicks in, check Tom & Angus’s book for details http://www.thechamonixbikebook.com/ (they list the trails you can ride legally) or http://chamonixbikeblog.wordpress.com/ for a locals perspective (look through the archives to what we were riding during July & August).
The lift pass is much less than 50e for the day UNLESS you want to go up the Midi or such like as well, when it is that much. There’s a selection of deals which Compagnie du Mont Blanc will get round to telling everyone about minutes before the summer season starts.
There are all manner of rumours at the moment as to what the bike situation will be for summer 2013, including that no bikes will be allowed on the Flegere or Grand Montets lifts. I’ve no idea what these are based on so until we know, what IS happening is a change from the Bellevue to the Prarion lift at Les Houches for the focus of the bike trails, including new trails getting built. More trails at Le Tour & the Flegere trails being completely abandoned with no bike access on the Index chairlift (the same situation as last summer, although the lifties were happy for you to maintain the trails yourself)
The bike ban’s been around for a few years, like it or not, it’s there and at least paying some attention to it and not riding high profile trails when chock full of walkers will help the situation not get worse. No VTT guide qualified to work in Chamonix will take you on the trails that are banned during July & August either.
Hope this helps, the riding’s great and you’ll not get bored in a week for sure. If you have transport then you’ve got an obscene amount of riding at your disposal in less than 60 mins driving. Enjoy!