People suggested a lottery but that isn't really a money sink, maybe you can add it in another way.
This is the original idea, price is 10m and for example 20ppl donate 10m in the lottery, that's 200m in total, 1 person will win the pot, that's how it suppose to go.
But what if there would be a tax on here too, 200m is the pot, someone wins it and the total of 200m will be reduced with like 1 to 10%, rather see 10% in my opinion or 1/2% less so they wouldn't win 200m but 200-20=180m.
The 20m will be permanently removed, this would be nice to see, also people will lose some money and no one will really notice that they've lost 20m from the total pot because donating 10m and winning 180m.... You wouldn't really notice that.
Something in that direction would be a nice feature.
Another suggestion would be a gambler npc somewhere where you can use an item on or minimum 10m and this npc can double it or you'll lose it , he'll be using like a dice, there would be a higher chance to lose than win to get the idea of some sort of money sink, also a chance to double the item/gp, not as big as losing though.
Let me know what you think, just throwing stuff out there for 'money sink' ideas.
Also feel free to add your own money sink idea.
-Sheep.