A Fleet Carrier with just the basic (Rearm, Refuel, Repair) costs only about half a billion per year to maintain. When you buy your Fleet Carrier, just allocate half a billion to the Carrier's account and you won't have to worry about it for a year.
You can earn a Fleet Carrier in only a couple of days if you work with the Pilots Trading Network and already have about a Billion in the bank for a trading investment. It works out to about 4 BN every two days or so. Do a couple of cycles (call it a week) and you have a quick 8-9 BN in the bank. Pay 5 for the FC, put another 1 BN in the Carrier's account (good for 2 years unattended), and you'll still have about 2-3 BN in "walking around" money.
Then, you can use your FC as a hauler for colonization or even just as a long distance transport for your fleet. I got completely maxed out on ALL my Mats in only a couple of days by visiting several G5 Brain Tree sites well outside the bubble. One FC jump to get there, one FC jump to get back. Trade all the G5 for lessers and then another trip out to the black to top off on the G5's just spent.
The FC hauls your fleet everywhere, it can haul them 500 LY in a single jump, it can haul them far out INSIDE a single system (like 400K LS) in that same single jump, and it can move over 24,000 T while doing all of that.
The FC is truly a game changer!