Pretty sure Costco does not have Alta or Snowbasin tickets. You can find discount tickets at most ski shops. For instance, Lift House at the BCC entrance has Alta tickets for $5 off, and Snowbasin tickets for $10 off. I just checked SeeNSki, and they have the same prices as Lift House.
I know you didn't ask about these places, but the last time I was at Costco they had the following:
Canyons -> $149.99 for 2 adult lift tickets (Normal price $105 per day)
Park City -> $144.99 for 2 adult lift tickets (Normal price $104 per day)
Snowbird -> $349.99 for a 5-pack (Normal price $92 per day)