Years ago I remember reading about a gentleman who decided to develop his own cruise missile based on commercial off the shelf parts, who was later shut down by the national security apparatus.
But a quick review of the SparkFun website (among others) shows that GPS enabled computer boards are extremely affordable, and can be coupled with other equally accessible propulsion systems. While the systems are not hardened against spoofing, the democratization of precision munitions is happening, at affordable price points.
Edit: He's a New Zealander who wrote about it in 2003.
The Missile Man - a book by Bruce Simpson