Upkeep is only half of the issue.
So, assume you have an FC... What then ? You still have to fuel it for the jumps to put it anywhere, and then there is still the repair costs..
FOR WHAT PURPOSE ?
Even if you got it where you wanted it 'in bubble', there are plenty of stations you can use within a few hops.. So why would I trade/repair at a loss ? Ammo or refuel ? I can synth what I need or scoop a star.. Outside the bubble ? Almost the same, synth and AFMU, solve most issues.. And I'm not making the trek to Sag A to sell LTDs..
The main thing I wanted was a mobile base I could drop 7k LYs from the bubble, map out every star in 200-400 LYs, sell my data ( or at least back it up ), and jump again. This would have been AWESOME in the search for Raxxla or searching the Rift. Jump out with 10 explorers, every week make a new jump. Imagine 4 or 5 FCs do this in a coordinated effort ? But as it stands now, every explorer on an exploration FC, would have to bring ( or re-outfit their current ) a miner, to search for fuel and LTDs, to help pay upkeep and fuel the next jump... Ugh... not what they wanted.
So, much like multi-crew, and PP, and Thargs... I'll pass.