You can find used C-Map's on Ebay. I found one for the area from Charlotte harbor to Key West for my Nav6000. It was under $25. We use paper charts when navaguessing, GPS is only for second opinion. We usually have 3 or more handheld GPS units for us to review and argue over who's is most accurate.
I installed a Raymarine 630 older color chartplotter in my Cal 9.2. It can relay seatalk data from the other instruments to popup windows for reference. Its nice to have the real depth displayed to compare to the chart's depth reference especially when the TVA drops the lake in the winter. We sail all yr and need to avoid the mud and rocks that the low water bring....
Cal 9.2 #19 SilverSwan