I think you're far too high in that estimate.
For a 9 to 4 day, the cost to operate one lift and minimally staff a base lodge comes in at around $1,000. Ignoring the benefits of increased season pass sales, you'd have to sell 30 day tickets at $35.00/day to cover your expenses. Even if my cocktail napkin calculation is off, I still don't see how you can get to your $5,000 to $10,000/day number.
There's several factors to it overall.
The first issue is the arrangements made to actually get a power for the lifts. Generally speaking, in order to get the juice flowing, the power company itself charges a fixed cost charge to have the juice turned on. Now, I'm not sure how their circuits work ON the mountain, but they aren't coming thru the same lines as say, the base lodge power is coming thru. I don't recall what mountain it was, but sometime, I think last year, at least one mountain had spoken up as to why they wheren't open, and it came down to having to make a 1 grand payment to actually have the power itself just turned on.
There are a whole slew of other things that could be taken into consideration. Taxes for the sheer pleasure of having the lifts running, pretty sure Vermont has that. Insurance costs. Trash removal.
In truth, you're right, the numbers 5-10k for a day is pretty high. I suppose it's a moot point, as we'll never know what it would cost them to stay open for an extra day. But I'd bet, as shewd businessmen, one of the major factors was, 'We won't make any money'. Is there unmeasurable bennies in marketting and customer satisfaction? Sure, a little.