You can buy a kit that works well for less than $350. Rostra makes them. I have one in my Dodge and I am getting ready to install one in my project truck. I am buying it from this dealer.
http://www.thecruisecontrolstore.com/ Tom seemed to be knowledgeable and was very helpful. I found them through an internet search and have never purchased anything from them.
The system is throttle cable based and it gets speed information from a sensor you place between your transmission and the speedo cable.