Fleet carrier, in my opinion, is a critical quality of life upgrade. For all intents and purposes, a FC is a space station that can jump (almost) wherever you want within 500LY every 20 minutes, with the main functional difference being that it won't buy stuff from you, but allows you to store it instead.
I mainly use mine as a drop-off point for whatever I'm doing. It is usually parked next to my favorite core mining site (my favorite pastime in this game), which allows me to get to the ring within 3 minutes of logging in and then drop off my stuff 3 minutes after leaving the ring. All my ships are always right next to me, I don't have to worry about pirates, jump range, what I'm mining, or how far I'll have to go to sell it. I just go to the ring, mine all cores that I come across and drop it all on the FC when I'm done. Then, after multiple trips, if I have a sizeable stock of a single mineral I just jump to a system that buys that mineral for a lot of moneys and make a single 5 minute trip in my hauling ship (python, to prevent bulk tax), returning with 200 million credits and tritium. Then I jump my FC back to the ring and voila.
If I compare that experience with how it went before I had my FC (fly across bubble to where my miner is, fly across bubble to a ring, pick and choose cores to only a single mineral, fly multiple jumps to where the mineral sells well dodging pirates along the way, fly multiple jumps to home base), the difference is monumental.
It also helps when you have a combat CG. Chances are that your combat ship doesn't have a good jump range and having to make 20 jumps to get from your home base to the CG system is annoying as hell, and satan forbid if you decide to switch to a different combat ship while you're there. FC fixes that, you just jump your entire fleet right next to where the CG is taking place (usually 1-2 jumps away from the CG system cause by the time you read about the CG and log in, the actual CG system will be full of carriers already xD). You just take off, pewpew to your heart's content, dock 3 minutes later and jump your entire fleet wherever you want to go next.
As for costs, my carrier has a "survival" setup. If I end up with my FC anywhere in the galaxy and lose my ship, the carrier has everything that would allow me to recover, including exploration ships, shields, FSDs, mining equipment and so on. I also have cartographer installed so I can make money in deep space. I just like the idea that whatever the hell happens and anywhere it happens, I'll be fine. This setup costs me some 35 million a week. Roughly once every 2-3 weeks I make a single 1hr long core mining trip, returning with 200t haul of mainly monazite and musgravite, worth no less than 100 million. Then, once every month or so, I bulk-sell whatever goes for best price at the time, making 200+ million in 30minutes including jumps, which covers the month of expenses.
FC is great. Go get one today.