At MoMA, the amount of revenue from admissions (almost $25 million a year, and these are all 2010 fiscal year numbers) is quite nearly equal to the amount approved by the board for yearly spending from their investments. (The museum overall has investments valued at $642 million. You know: 2/3rds of a billion dollars.)
As well, that number is also almost exactly equal to the amount of money the museum spends on curatorial services alone—not even including exhibition costs.
Museums in this way are like newspapers (and maybe colleges as well): the subscribers to a newspaper pay for some of what they "see"—the words and pictures—but don't even begin to cover the cost of actually publishing a newspaper. Those customers just can't be, in most cases, where the money comes from.
Annnd that's (sort of!) why newspapers have advertising. (They also have advertising because "they can.") In these models, there's only so much you can squeeze out of the customer base. The museum is now overstepping its bounds in its relationship with its visitors, as it raises the admission price from $20 to $25. (It should be noted too that the price went to $20 just back in 2004.)
But someone went and did the math about raising admission and membership prices, and calculated that, despite surely losing some members, the projected income still goes up a bit. And then the administrators get to say they're doing their job to protect the museum's future with that little uptick out of visitor's pockets—while the rest of us only visit the museum on the Target-sponsored free admission Friday evenings.
It's interesting too that in 2010 the museum made almost as much money getting rid of art ($11 million—about half the annual income from admissions!) as they did buying art (almost $16 million).