Typically when a government program talks about returns on dollars spent it’s not so much about the profitability (government walking away with more dollars than they put in) as much as community benefits that lead to cost savings within the greater economy. In this case the return is in health cost savings, as parks are a place community members can go to exercise for free, directly improving their physical health and therefore reducing their need for healthcare, saving money spent on healthcare for the community as a whole.