whitttt????? 5 billion??? is that the price of a fleet carrier? jeez......................................

If you want one, it's a goal you can set yourself - and it's entirey doable. If you have the time. And the willingness to grind for credits. Plus, you'd probably need another 1bnCR to install the services you want and be comfortable.
Of course, after what
@nemolomen says, there's fuelling the blighter. About 700 tons of tritium roughly translates to about 135mCR. A full depot of tritium is 1,000 tons. Spare tritium - and you will probbaly need it - is stored as part of the FC's overall inventory.
And don't get me started on getting fuel from your inventory to your depot
All doom and gloom aside, you can - and probably will - get the cash together to get one. And once you do, you won't look back. Probably *
* Note the overuse of "probably" in this post.