they call it the lone peak pass, and for a day for a "junior" (11-17) its 69 bucks. 89 for adults. as aenigma said, its not really worth it for one day. its not really possible to ski all that terrain in one day.
I think that the only reason that it would really be worth it is if you want to ski some of the chutes on the moonlight side that are only really accessible from the tram or the big sky chutes that are only assessable from moonlight. But when we were there over xmas we ended up just skiing moonlight because its only like $40 with a student id and some of the hikeable chutes just from moonlight are just amazing... I'd really like to try big sky after a big dump but moonlight is so cheap its just hard to pay that much more.
President's day=packed. That's about all I have to say about that.
As far as the summit snow field goes, I wouldn't buy a ticket just to ski that. Yeah, it's an amazing run, but it's only open sporadically and for a few hours a day. You're setting yourself up for disappointment unless you can get a hold of patrol that morning to see if it's going to be open.
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