The cheapest FF I have is a Lowrance Hook2-5 HDI with mapping, trad. sonar, and DI. It came with Navionics maps. About $299. I use it on my kayak.
When I got a 17' boat, I bought a Garmin Echomap+ 93sv. Earliy in 2019, Bass Pro and Cabelas were blowing them out for $599. It has pretty good mapping (Garmin owns Navionics), and SI, DI, and trad. sonar. I don't think any are left at this price.
I just upgraded the 17' to a Humminbird Helix 9 MSI G3N because HB has the best SI IMO. The Garmin will be moved to the bow to run a Panoptix LS system. Then I will be flat broke, but will have an awesome fish finding setup. IN MY OPINION.
I find I use the mapping and SI the most, because mostly I am looking for fish. I think you want both if you can get it. Maybe find a used Helix G2 with SI, and stick a lakemaster or navionics card in it.
Amazon has a simrad go5 with the totalscan transducer for $505.
They have the 7 inch one for $569. With the big transducer.
I have one and like it. The side scan is awesome. It will hold bottom at 45 mph and mark fish at 20 and it will network with all the other stuff on your boat.
The above recommendations are certainly all great choices. I highly suggest looking into getting a unit and Navionics card bundle. You will really regret not getting a map card with your unit and will end up spending more money later. Heres a link to some bundle units: