I think a docking fee is a good way to have a credit sink in game.
You only pay when you dock, if you don't play for 3 weeks it doesn't matter as it is only levied on docking.
BUT, The challenge is that starting players with 1000Cr might find even 10Cr fee difficult to meet.
So do you say Sidey's have zero docking fees as it's covered off by Pilots Fed membership, that way as long as you are in a sidey you don't incur docking fees? Or do you say only medium and large pad ships have a docking fee associated.
Is it a flat fee, or a percentage of ships value, or percentage of transactions made whilst docked (kind of a sales/mission collection tax)?
The question then is for these Billionaires, whatever you levy it doesn't matter, for those that are just Millionaires again it may not affect them if the charges are equivalent to a few percentage points on any transaction. Then for start ups these charges amy also be unoticeably small if it is a percentage, or become more challenging if there is a minimum fee regardless.
You could double or triple fuel price instead, more people may fuel scoop, but even tripling fuel costs wouldnt break the bank of players, even start ups?
I would turn it to the behavior an airport or port is acting today. They don't charge for docking events. They charge for time being there (wall time). Soemthing that would show
how a station can manage to be operated. Prices for wall time can differ depending on station, traffic, government, system status and so on. Stored ship can be handled differently
than those "on pad" which will help to avoid the existence of pad blockers. If something starts to create cost everyone tries to avoid it.
Regards,
Miklos