These studies usually give economics a bad name:
We should be skeptical of “economic impact” studies that show the importance of the arts to a community. A study of this kind might show that an arts festival or new arts arena brings millions of dollars in economic value. But these studies typically treat arts expenditures as creating value out of nothing. Implicitly it is assumed that if the money had not been spent on the arts, no other economic or social values would have been produced. Again, the relevant comparison is whether an arts arena leads to more value than some alternative. When we look at economic impact studies for one industry at a time, they all appear to show high benefits. But this means that the net benefits of any single project are low, zero, or perhaps even negative on average. By investing in one good idea we are always forsaking another good idea. In essence those studies list gross benefits rather than net benefits. Furthermore, once an economic impact study is being done, the resources are likely no longer undervalued.
That is from my Good and Plenty: The Creative Successes of American Arts Funding.