Wind energy becomes a feasible option when a site has average annual wind speeds of 8 MPH or greater. Typical wind speeds increase 50% or more at elevations 100 feet above the ground. Turbines must be 20 feet above any tree and unobstructed for at least 200 feet. Typical output of a 3 foot rotor in an area with an average annual wind speed of 10MPH is 200 kWh/yr (kilowatt hours per year) whereas the larger 10 foot rotor will produce 1800 kWh/yr in the same location.
Here are some typical rotor/generator prices...