This along with Wealthy is not accumulative as the descriptions would suggest rather you need to be holding 100,000 thousand lucre at once for it to unlock. Good news is it all carries over across NG+ and since you need to do 3 playthroughs anyway this achievement will come soon enough.
Don't forget to sell items and gear you no longer use and if you really can't wait and are close enough you can always save, sell EVERYTHING then reload your save after popping the achievement
Farming is not necessary.