An analysis of neighborhoods surrounding three greenbelt areas in Boulder, Colorado, indicated a $4.20 decrease in the price of residential properties for every foot moved away from greenbelts.
Correll, Mark R., Jane H. Lillydahl, and Larry D. Singell. (1978). The effect of greenbelts on residential property values: Some findings on the political economy of open space. Land Economics 54(2), 207-217.