Shared mini buses run regularly from Geneva airport in the Winter and Summer seasons. Most stop in Sallaches the the Valley on the way to Chamonix, then it’s a 25-30 min drive to Praz – with a taxi (local taxis here) or public bus (see timetable here). Some transfers may, in the winter, run all the way to Megéve which is just 10 mins away (try here).
Shared transfer costs around €20-30 per person one way. A taxi by contrast may cost €200-300 but if your a larger group (up to 8 in a van) it might make sense spending a bit more for the convenience of being taken right to the door.
Check out BusBud for options to find a shared buses from several operators.
Try GetTransfer to find the best fare for a private transfer – may take a couple of days for the best offer to come through
Stuart reports that a seat in this shared coach to Chamonix is only 29euro and will drop you at the MacDonalds in Sallanches (near Carrefore)
http://www.chamexpress.com/
Beware Chamexpress turns out to be nearer 40euro when you factor in options and taxes