How much do fleet carriers cost in 2021?

The cost of a Drake-class Fleet Carrier (only class available; aside from cosmetic reskins) is 5 billion (5,000,000,000) credits. Optional facilities (outfitting, shipyard, etc) have to be installed separately; the total cost of all optional services is about 1.1 billion credits.

The weekly upkeep depends on what services it provides, but it ranges from a minimum of 5,000,000 to a maximum of 26,700,000 if all optional services are active. A service can be temporarily suspended, which will reduce its upkeep (approximately by 40-70%, depending on the service). It can also be removed entirely, but the installation cost is only partially refunded and has to be paid in full to install it again.

If the upkeep cost isn't covered, the fleet carrier goes into debt. While it's in debt, all services are suspended (reducing their upkeep). If the debt exceeds 250,000,000 and isn't paid within a week, the carrier is decommissioned. The carrier can also be manually decommissioned without being in debt; this costs 125,000,000. The full price of the carrier and all services (minus debt) is then refunded. The player can then buy a new carrier any time.

(Source: In-game info, as well as this wiki. https://elite-dangerous.fandom.com/wiki/Drake-Class_Carrier)