I tend to have to spend all of what little time I have grinding for upkeep. This is an unfortunate consequence of having a fleet carrier.
I wish there was a way to make it a bit more self-sustaining.
I've tried for months to get a carrier to be passively self-sustaining with simple things, but realistically that's not been viable.
You'll probably want to source commodities and materials for which there's a heavy player demand - and sell those on your carrier.
So that'd be Meta-alloys, Modular Terminals, and Caustic Tissue Samples for the commodity market.
And it's Power Regulators, Suit Schematics, Weapon Schematics, and Ionised Gas for the bartender.
Price them all ten-thousand percent, but keep the Meta-alloys at something like five million credits per unit.
Regulators and Suit Schematics sell for 5M¢ each, with Ionised Gas and Weapon Schematics selling for 3½M¢ each.
Of course, you'd be responsible for sourcing the materials yourself. And, critically, you'd need to regularly update your carrier's market data on the EDDN with a utility like the Elite Dangerous Market Connector -- and you'd need a properly set up Inara account to boot, since that's how other players would find your carrier. You'd need to log into the game for like two minutes a day to update your carrier on EDDN.
If you hit Arai's Mine (please don't, just source missions and do those) you get probably two schematics, one suit and one weapon. And you pull the regulator. That's 13½M¢ for an Arai run, on average.
There are much better settlements out there that'll net you an average of ~25M¢ in player-desired materials for a single run.
I'd recommend you sell directly to other player bartenders and skip the EDDN thing -
just search the Bartender demand on Inara.
Just don't forget to sort on Price - sell at premium pricepoints only!
Update 16 has been a beautiful thing - it's really opened up the player economy.
Another question: What's the benefit of upkeep? Some people defend it very loudly.
There is no direct benefit to players.
Never has been.
The indirect benefit is that it really works to get rid of carriers - I've seen a lot of 'em go away.