Hi Egbok,
I was just browsing the unread messages and saw yours, and thought, this is one i can answer - my DH and I actually run a catered chalet company in Les Gets!
Re the cost, that week is THE most expensive week. It's hard because you're tied to school hols, but if you can go at Xmas it'd prop be a bit cheaper (though the snow is less sure then). And Easter would be cheaper still. Per person rates will be in the region of £600-700, but this should include your food, breakfast, afternoon tea and 3 or 4 course evening meal, or high tea for kids), accomm and transfers. You should get a discount for your children too, paying 20-50% of the adult price.
To save money, if transfers are included in the price of your holiday then you should be able to get money off. Ski hire and lessons and passes are all pricey - ?150 for an adult pass, a bit less for adult ski and boot hire, and the lessons for 5 mornings are I think about ?130.
You may find that places are already booked for half term - people reserve spaces in chalets reeeeally early. It might be worth thinking about going self catered. I'm not sure about prices, but the Les Gets website will have some info on this:
www.lesgets.com/en/votre-hebergement/index.html
Unfortunately, skiing is an expensive holiday, and even more so for families... it's great fun though, isn't it?!