Economic impact studies have been used for decades to estimate both the direct and indirect effects of money spent in the arts, in the nonprofit sector, or for other activities. They have playd an especially prominent role in debates about whether or not to fund new public facilities like sports stadiums.
Some useful links include:
The American Prospect published a critical, useful and accessible essay on economic impact studies in the arts in 1999:
http://www.prospect.org/web/page.ww?section=root&name=ViewPrint&articleId=4498
Americans for the Arts, in particular, has done a lot of work in this area. See, for example:
http://www.americansforthearts.org/information_resources/research_information/services/economic_impact/default.asp
NCNA links to various state nonprofit reports:
http://www.ncna.org/index.cfm?fuseaction=Page.viewPage&pageId=327
Michigan nonprofit sector study:
http://www.nonprofitresearch.org/newsletter1531/newsletter_show.htm?doc_id=31229
http://www.mlive.com/mbusinessreview/west/index.ssf?/base/test/1165496408312970.xml&coll=12