The power a wind turbine will extract is related to the area of the turbine and the wind speed, plus an efficiency factor since no turbine is 100% efficient.

There's a good table relating power (watts) to wind speed (mph) and turbine area (square feet) at

http://www.ndsu.edu/ndsu/klemen/Perfect_Turbine.htm
A few data points ...

5 MPH wind, "good turbine" (for efficiency) will deliver 0.2 watts per square foot of swept area.

10 MPH wind, 1.75 watts per square foot

15 MPH wind, 5.9 watts per square foot

As a guess, a typical rooftop exhaust vent turbine will have an area of 1 or 2 square ft. Let's say 1.5 square ft.

Since the power produced is not linear with wind speed, multiplying these numbers by the average wind speed will not produce accurate results. Fortunately, the referenced web page has calculated monthly energy production using a Rayleigh distribution of wind speed.

If the average wind speed is 10 MPH, which may or may not be representative of your location, you will generate 2KWH a month of electricity with a "good" design turbine per square foot of turbine area. (There's a map of estimated wind speed for the US at

http://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf. However, the wind speed data is for 50 meters (165 ft) above ground level, and typical house height wind speed will be much lower unless you live on top of a tall ridge.)

We've assumed 1.5 square ft for the projected area of a typical roof ventilation turbine, so that's 3 KWH per month for one turbine, hooked to a generator.

Here in northern Virginia, we pay a bit under 10 cents/KWH for power from the NOVEC co-op that serves our area.

So, you would generate 30 cents worth of electricity a month based on all these assumptions. Feel free to plug in more accurate numbers.

In my view, the only reason to make a turbine generator at this scale would be for amusement; it certainly is not a cost effective way of generating anything more than a trivial amount of electrical power. A small solar cell array would be more useful.

Jack