If you’re looking for a basic, monochrome fish finder at an affordable price, that also has GPS, the PiranhaMax 176i may be exactly what you need. This unit has a 4-inch diagonal display, 16 Level Grayscale, with a 240V x 160H pixel resolution and backlight.
It has a depth capability of 600 ft., having a power output of 200 Watts (RMS) / 1600 Watts (PTP). It has Dual Beam sonar technology, therefore it can operate at two frequencies: 200 kHz, with a 28° view, and 455 kHz, with a narrower, 16° view. It has the Fish ID+ sonar view function, which allows you to distinguish better between fish and structure, and see the exact depth of the fish. The standard transducer that comes with this fish finder is a Transom XNT-9-28-T.
The PiranhaMax 176i also has internal GPS antena, and a few other basic navigational features such as trackplotting, being able to display up to 2,000 track points. It can store up to 50 waypoints, and also has water temperature.