If you are lucky enough to be going to the Three Valleys this year, I'm jealous.
The first year I went, another rider in my shuttle van terrified me with stories of $100 lunches. While that's certainly possible (I did have an €35 hamburger at Le Cave de Creux once), there are some cheaper options
There's a very key comment in that article: They have a snack bar just underneath the main restaurant if you want to grab a quick bite on the go.
That's a key thing to remember -- lots of restaurants will have a snack bar window as well. (I generally find them under the sun deck, if they have one.) It's a snack bar with picnic tables, but French hot dogs are REALLY good, and you can't beat the view. And you get back to skiing quickly. And you have more money for tartiflette for dinner.
I can also recommend the snack bar at the base of the Pralong lift in Courchevel. A huge sandwich (enough for two skiers) for €10 the last time I was there.