$850 will get you a used GPS/si/di unit in a 9 inch size, probably a HDS Gen 3 on BBC. That's only three generations back from current offering. However I'm sure the markup on new units is substantial for the manufacturer's and for the dealers they have MAP pricing, so they get their cut.
They may still be shallow right now, caught three last week that made us a batch of fish tacos. Real shallow and in one spot, I was fishing for bait actually. The better ones are usually out deeper, like top_dollar says.
I don't know if that unit has CHIRP but 83 Khz would be a wide(r) view in normal 2D sonar. You'll get more returns but they might be further away from the boat. I think my HDS units run 2d at either 200 or 50 KHZ.