Yea a lot of people forget not everyone has 8 billion lying about...
My CMDR has grossed about 5 billion in income in 6500 hours of play time.
I'd still consider this to be a bit on the low side of ideal for a fleet carrier. Of course, my idea of ideal would drive most players to dispair, possibly suicide as it would involved ten bliion credit, uninsurable, completely persistent, and destructible, fleet carriers....precisely for the purpose of bankrupting as many CMDRs as rapidly as possible so that the economy could be implemented/corrected.
There is not a chance in the world they will cost more than 2bn. This is easily justifiable as it's like the difference in cost between say, a top of the range personal watercraft (small yacht) and a container ship. The yacht is, for example representing a fully decked out cutter, and the container ship represents the fleet carrier obviously. These two costs in real life are about the same for what I consider to be scale equivalents (around $10m). One is simply a whole different scale of usability and ergonomics and the other is RIDONCULOUSLY far bigger and could store 20 of the other. Size does not equal cost. So based on that, a fully decked out Cutter is 1bn, so the fleet carrier should be around that figure, too.
While I also expect fleet carriers to be entirely too accessible, I don't agree with your analogy.
A container ship is a big floating box with an engine. A carrier is a complex mobile air base and command-control facility that can indepenently support an entire air wing (or small fleet, in this case) for protracted periods of time. Size also heavily imples cost, when the overall technologies and quality is similar.
A cutter is also not the equivalent of a small yatch. An FDL is the equivalent of a small luxury yatch. A cutter is a mega yacht or small frigate.