Don't expect too much from terrain parks in France... There are some great ones but even the best are far from what you could find in Switzerland or Austria. If you don't want to ever go out of the park, there are better options in Europe. That said, France probably have some of the best, and biggest, resorts in Europe. The ski passes aren't the expensive part, it's housing.
Your best bet for a season is to look at living in big towns/villages further down the valley, or find a job that offers accomodation for the employees.
If it is for a few days/weeks, avoid february like the plague. It's the peak of the high season and everything gets quite expensive. I could've said that's when there are the most parties (which is something resorts in France do really well) but this season it's pretty much cancelled. Plus most parks aren't completely built at that time, so you can be up to quite some disappointment if you want to ski park before march. Many resorts offer a discount if you rent room/appartment there. There are a lot of discounts available throughout the season, but it can be hard to find them.