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.
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: