Bunkers. Similar to what Israel said, a kind of bunker esque station in a certain area or PVP hotspot to make it worthwhile.
The most success we’ve ever had at creating a massive money sink was with the Bunker system. Sure there was drama, but that could all be solved with a strict set of rules when it comes to using/attacking/defending bunkers so that people aren’t doing any of the goofy stuff they did before. The penalty for this would be the loss of their bunker, no refunds and no buybacks.
For those wondering just how successful the bunker sales were, check this out:
(Also make it clear that those bidding can only bid for version 1 and version 2 and can’t swap from version 1 to version 2 when version 1 is too expensive like RED/OPG did which seemed a little bit sneaky 
! Otherwise they can inflate the price of Bunker A and then switch over to Bunker B which is somewhat cheesing an auction.)
But all cheekyness aside, the total amount taken from the bunker sales was around 1 billion credits!
So it’s probably one of the most powerful money sinks ever created!
Just don’t let people edit or modify their bids halfway through or it causes chaos but you picked up on that in the last one and forced the other parties to play fair! It was good fun!
Best regards,
Wise.