[the following is by, Robing Dropkin, Exec. Director at Parks Trails NY ]
As a park advocacy group, we, of course, intuitively recognize the importance of parks. But we also wanted some hard facts. So we commissioned a study to measure the impact our renewed park system is having on the state’s economy. Turns out the increased investment in parks is reaping big returns.  
The independent study by the Political Economy Research Institute of the University of Massachusetts found that spending by State Parks and its visitors supports $5 billion in output and sales, 54,000 private-sector jobs and more than $2.8 billion in additional state GDP.
In short, every $1 invested by the state leads to $9 in economic activity. That’s a great investment anyway you look at it!. Read the report here.