Museum of Fine Arts in Boston is 25.00 for adults although they have a free times as well.
Boston Aquarium is currently 19.95 but is usually 22.95 for an adult.
Those are just the first 2 that I could think of locally. Seems like it's the going rate in a major northeast city.
Locally here the Academy of Sciences is $30, DeYoung is $27 if you view a special exhibit, SFMOMA is $24 if you view a special exhibit, Asian Art is $22 on a weekend if you view a special exhibit, Aquarium of the Bay is $18 (and it's a total rip-off!) and Exploratorium is $25 (and it's totally worth it!) They all offer free admission on certain days, most offer discounts if you use public transit & present a valid pass, and you can also purchase "CityPass" or "Go S.F." cards that give you discounted admissions to multiple venues. I think they probably bump up general admission prices because they anticipate a certain level of discounts.