Simple and cheap would probably be one of the PiranhaMAX's by Humminbird for around $100. My thoughts are that you get the most in a graph that you can afford and emphasize on screen size and quality. Most are simple and all you really have to do is turn them on and go. No special setups to make them work. Save a little and maybe go used to get more.
Couldn’t say it any better then Todd did.
I would add that before you buy one you visit a store that has several in display. Remember you will see them a lot better in the store without the sun glare.