I really doubt you will find a used GPS with mapping for $50 or under. A GPS won't find structure for you, other than bottom contours. Brushpiles, old roadbeds, bridges, house foundations, or other structures that were covered when newly built lakes are flooded, probably aren't on a map chip. Things like that you can find if there are old topo maps of the land before the lake was built. You can get the coordinates off the old maps, and then find them with a regular GPS. If you find a regular GPS that is Waas (more accurate) capable, just cruise your favorite water with your depth finder, then mark any structure you see. I'd suggest some marker bouys so that as you pass over structure, throw one out, then go back over the point and throw another bouy. Your actual structure should be between the 2 bouys.