Upkeep cost implies you want to have people make periodic payments. Do you want payments per real time, or payments per time played? Or payments per cap flight time?
Real time puts disproportional pressure on people who have less time to play, as they would be required to devote much more of their playtime to the upkeep of their capital ship than someone who plays daily. If you want people who play daily to feel pressure from your capital upkeep, this would make it impossible for someone who plays a couple of hours a month to maintain a capital ship even though they pay for the same game and have spent as much playtime as other players to buy that ship. Real-time upkeep puts disproportional pressure on people with less playtime.
Thus, real-time based payments are out.
Next, time-played upkeep costs. Would it be based on character logged in time?
In this case you’re severely punishing main characters that can also fly a capital ship, but spend a lot of time logged in not flying caps. You would force every capital pilot to be an alt for monetary reasons. I sure would be incredibly annoyed that I cannot use my fax alt for Porpoise boosting anymore when the extra upkeep payments for logging in that alt negate the profit of having a Porpoise in space. Punishing a pilot for being logged in just because they have a capital ship is disproportional for people who use characters for more than just flying that capital ship.
Thus, played-time payments are out.
The two previous upkeep options aren’t any good, but perhaps time spent inside the capital is a better measurement? After all, this would be fair for everyone, the people who play daily as well as those once a mont, the people who fly capitals on their alts as well as those who also fly capitals on their mains!
How can CCP easily track time spent inside a capital ship? I have an idea: capital ships move around when they’re used. Most often, they make use of their capital jump drives to do so.
Maybe CCP could put an upkeep tax on capital movement? Something like a travel tax?
Call it… fuel?