The question is, what do you want it to do? If its tell you where you are on a map and how much water is below you then a basic unit will do just that. If you want a fish finder, sonar or high resolution down scanner, then you pay more. Also, how much console space do you have? do you want to connect to a DSC radio or AIS receiver?
I have the Garmin Echomap 50dv. Very simple to use and good quality sonar images. I bought this one because it has more NMEA in/outputs than the similar Lowrance and Raymarine units so would talk to the DSC/AIS radio and the smartcraft gauges. It also has high-res down-scanning sonar for picking out under water features and wrecks to dive. Most makes are pretty good these days. Just buy to your requirements and budget.