As Beefy says, for non CAF members the refuges are generally in the order of 40-80 euros per night including dinner and breakfast. If you are a CAF member or a member of a UIAA affiliated club then it's around like half of that. If you're not already a member of the New Zealand Alpine club then it's worth joining just for this, plus the gardiens just seem to treat you nicer. There are also some small bivvys that don't have a gardien and are free but are generally grotty as a result. For any of the popular huts over summer you will almost certainly have to book well ahead of time.
If you are on a tighter budget, another option is to camp/bivy. You can pitch a tent above the snowline in France between 7pm and 7am or thereabouts. Some poms at the col du midi were leaving their tents up permanently but there was talk about cracking down on this last year. If you do bivy, try to stay a fair way from the huts to keep the peace (the gardiens make their livelihood from providing meals). You should be able to do this on the Italian side as well. Because access is so bloody easy from all around the massif, it's easy to wait out the bad weather in Chamonix then head up high and bivy for several days if there's a reasonably stable forecast of good weather.