I agree wholeheartedly with Julia that economic impact (or any impact for that matter) is very hard to tie conclusively to public art.  She is absolutely right about the study design that would be required, but even then neighborhoods are so complex and there are many factors that are outside the control of a project.  I used comparison sites in my evaluation of public art projects (not focused on economic impact) in LA County and ultimately we couldn't control for all community factors (like violent crime).  If you are interested in what a pre/post analysis with comparison sites might look like you can read the evaluation report here: https://www.lacountyarts.org/article/report-art-infrastructure

But I would also echo Julia's point about being super clear about what specific impact a project is intended to have.  If you are very clear about that, you would probably have better luck making a case for the impact you are intending (as opposed to conclusively proving it) by a rigorous qualitative design.  If you are looking for some examples of economic impact, there are some more nuanced discussions of public art/creative placemaking in the context of economic development goals in this report: https://metrisarts.com/studies-culture-in-community-development/#storefronts