Well I'm currently running 3 carriers in Live with another 4 mothballed on console/Legacy with a decade of upkeep each.
Upkeep per carrier is around 20 million per week. I can make that without even considering it.
Mine costs 19.5 per week. A minimum carrier with refuel, repair and restock costs 9.5 MCr. In the grand scheme, that is nothing. Killing a single Orthrus finances a minimum carrier for four weeks. A Python-load of platinum sells for 50 million easily, a Cutter-load for 175. If you are into core mining, some of those still sell for over half a million per ton, so a Python full of those will get you around 100 million. First discovery of some organisms get you just shy of 100 million for just
one organism. Combining an hour of bounty hunting with the odd massacre mission earns me upwards of 60 million. A single high on foot CZ can bring up to ten million for 15 minutes of pew pew.
Credits are really easy to come by once you know all the loops and do it at least somewhat regularly. I parked all the credits I made from getting into AX on my carrier because I didn't know what else to do with them, and it is paid for for the next seven years. It is not rocket science.
Having said that, people need to accept that a carrier isn't for beginner and/or fresh CMDRs, and not for those who dabble into the game for a few hours every few months. That's not trying to be condescending or gatekeeping or anything; those are just not the target audience for owning (well leasing) your own megaship. I don't know why this is so hard to comprehend. I stand by my sentiment: If upkeep worries you, you're not there yet.
Maybe that makes me Scrooge McDuck. But if I am, then so is almost everyone.