You may want to consider the cheap $80-$150 baseball-style radar guns (like the Bushnell Speedster).
I bought a Bushnell Speedster and needed to fly the plane right over my buddy's head (probably about 20 feet above the ground) in order to get a reading. The max range (line of sight) at which I could get a reading was 30 feet. My buddy was able to get a 121 MPH reading on my MVVS .40 Diamond Dust, after many attempts. The police-style radar guns (like the Stalker), give you the advantage of being much further away from the target but are almost 10 times the price..
The Bushnell guns do work, but you need a very brave soul indeed to stand underneath the path of a Extreme Speed Prop Machine.
-Mike
Here's an interesting web-page with many kinds of radar gun.
http://www.opticsplanet.net/radar-guns.html
This one looks pretty cool
http://www.opticsplanet.net/sports-r...un-sr3500.html