I haven’t bought one (yet) but Mattel is selling a toy RADAR Gun for about $25. The idea is to measure the speed of Hot Wheel cars or your buddies on bicycles. At first I thought this couldn’t possibly be real RADAR, but the box clearly says 10.5 GHz!

A little searching on the net found this Australian post that has internal pictures and discussion about hacking and modifications.

I’m sure for $25 it isn’t going to compete performance-wise with a “real” gun, but it looks like a cheap source of parts for a 10GHz oscillator and detector, along with a PIC FFT circuit!