"Putting a Value on State Parks"

Return on investment:

Each year, more than 700 million visits are made to America’s 6,600 state parks. ... Using conventional economic approaches to estimate the value of recreation time combined with relatively conservative assumptions, the estimated an annual contribution of the state park system is around $14 billion. That value is considerably larger than the annual operation and management costs of state parks.

That's "about $62 per person annually, on average." More here.