In my game, Strongholds, a bunch of adventurers brave the dungeon to come out with as much gold as possible.

Each adventurer gets XP equal to the amount of gold the whole party carries out, and eventually they'll have enough gold to buy their own stronghold.

I have an economy, obviously, and I believe I have a problem.

I have a sink for low amounts of gold - consumables, equipment - and a sink for huge amounts of gold - the stronghold. However, while the game requires spanning at least five orders of magnitude in currency (from single coppers, to tens of platinum coins each worth 1'000 coppers) there is no sink for medium amounts of currency.

I considered, but discarded, high-cost items with small incremental bonuses, because I don't think they would be very interesting and I don't like the added mechanical complexity.

I also considered high-cost magic items, but this would require me to structure magic items in tiers of price, while there is no inherent ranking now.

Any suggestions?