The best deal at Big Sky is to get a frequency card. If you buy them early they cost $35 & give you $15 off each daily ticket. Unfortunately, you are too late to get one at $35 (they cost $75 now).
Your best option now is to see if where you are staying has a package deal for lodging and tickets. The other ways to save are to bring along student IDs if your group has any and to take advantage of kids 10 & under skiing free (you can have up to two free kids tickets per adult ticket).
Skiing a day or two at Moonlight Basin might be a way to save money. Its a new ski area on the north side of Lone Mountain. Tickets there cost $34 for the one day but you can get $5 each successive day. There should be some posts about how good the skiing is at Moonlight before your trip.