Snowbird numbers are much lower than Mammoth, but the proportion that are day ticket buyers rates to be higher. The late season Snowbird skiers may be locals, but they can be passholders to other areas, Alta in particular. Snowbird does not do nearly the amount of spring work on the mountain that Mammoth does in terms of number of lifts, being open 3 days a week vs. 7, grooming, salting, moving snow around if necessary. So I have to believe Snowbird's operating expenses are lower.
The Memorial Day numbers snowboard247 posted showed Mammoth visits to be 2/3 passholders. Mammoth does get some northern California business after Tahoe shuts down, but not a huge amount. It's still a 7+ hour drive from the Bay area until Tioga opens. Mammoth has nearly always hosted racers in late May/early June.
I suspect late season at A-Basin is quite profitable. I get the impression there are even considerable weekend liftlines, though lifts are slow and capacity is much less than Mammoth. And as in Utah they can be selling day tickets to people who skied other areas mid-season, though in the current era there are probably a lot of Epic/Colorado passholders.