Must have 50,000G ON YOU.
You'll earn this naturally as you play by shipping crops, animal produce, or gems from the lake cave during the winter.
Another easy, albeit cheesy way to get this and the other money achievements is to save up about 5,000G (more the better) and wait for the Derby to come around (Spring 18 or Autumn 18), save BEFORE entering the Plaza, and betting everything on a horse with a high multipler.
If you lose, reload your save.
If you win, buy as many Brooches as you can, and sell them the next day (shipping box or Huang).
I bought about 170 and earned earned almost 300,000 the next day after I shipped them.