This should be quick and easy to do and would be a relatively entertaining infinite gold sink.

There's a million ways to do it but keeping things simple would be fine. Make a NPC that sells a lottery scratcher ticket or something along those lines for different prices points; something for everybody regardless of wealth. Maybe 1k, 10k, 50k, 100k, 1M, 10M.

Make it like IRL scratchers where the odds are pretty good to win something, but at the end of the day slowly funnels cash away from the game.

Like, let's say you spend 1M GP on 50K GP tickets. 20 tickets. Most of the time you won't win a big jackpot, but you'll most likely win around 400-500k back out of your 1M spent. This keeps people enticed to keep playing because eventually you'll hit something decent and come back for more. BUT, at the end of the day; with the jackpot prizes being paid out occasionally it should be programmed to average out where only 60-70% or so of the GP spent is won back by the players. Most of this in the smaller payouts, but of course occasionally the jackpot will jump out at a lucky player.

The odds would have to be carefully calculated for this to work, but if any admins or devs have even a slight interest in doing something like this I could easily do the math in a few hours and post it.