On the Trimble side of things the EZ-guide 250 is a very nice entry level system, simple to use and easy to install and move to other pieces of equipment.

Here is a nice video showing a simple little demo for the monitor, so you can see how things are operated and the structure of how things set-up

http://www.youtube.com/watch?v=MGZaH0XiV18&list=PLfnkNdV9GAfGtt1pUV8XwxIWjugLbD9OR&index=8

A very large consideration when purchasing precision products is dealer level of service and knowledge, make sure they have the ability to repair/service and train you properly on how to use this technology, I know this is a simple system, but if they teach you some tips and tricks on using it, it will accelerate the pay back on the purchase

 Do you have trees next to your fields? If you have tree groves, best keep your foam marker installed and maintained. Otherwise the "cheapest" gps will be even more frustrating than a poorly performing foam marker. I still drop foam on the head lands while using my cruiser 2. By then, I am far enough out from the trees, I can usually do ok with just the gps using the A/B line mode.