Skiing at Big Sky [1] is not cheap, but there is an enormous amount of terrain here. For those who just can’t get enough add Moonlight Basin [2] with a Big Sky–Moonlight Basin combination pass. It’s $93 for adults, $83 for seniors and college students with ID, and $73 for youth ages 11–17.
Links:
[1] http://www.moon.com/destinations/montana/the-missouri-headwaters-and-south-central-montana/big-sky-and-the-gallatin-valley/s/big-sky
[2] http://www.moon.com/destinations/montana/the-missouri-headwaters-and-south-central-montana/big-sky-and-the-gallatin-valley/s/moonlight-basin