The Garmin 140 is a good resolution 5" diagonal display in black and white that goes for the $200 mark. Basic and easy to use and customise and much better than using a screen 1/2 the size even if it is colour (colour models start just under $400)
They also have a good solid mount so you can leave them mounted to save the electical connector damages. Mine remains mounted on my yak for the past 18 months without a drama and transported upside down on the racks.
You can not do that with humminbirds or eagles as thier mounts do not cope with it. The more you unplug and replug the connectors the more wear you get and more chances of damaging something. I assemble mine first up with dewatering fluid and marine grease to keep the water out.