The Institute of Museum and Library Services has released Exhibiting Public Value: Government Funding for Museums in the United States
Here's an excerpt:
As a proportion of total revenue, government support to U.S. museums ranged between 7% and 33% by museum type. However, when government administered museums are removed from the analysis, the highest proportion of public support drops to 24%. A great deal of variation lies beneath this simple estimate. Because museums of different types have widely varied operating budgets, equivalent proportions represent radically different public dollar investments. For example, while science and technology centers and history museums reported similar proportions of their operating support coming from government sources, 30% and 32% respectively, the public dollar investment is quite different. For science and technology centers in the survey, the median public support was $289,970. For history museums, the median support was $32,182.