An easy"ish" way to get this achievement is to beat Magic Man's story and unlock the Moonstone Dispenser. Then set a perpetually growing farm of moonstone. Moonstone sells for 500 (1.5x more if sold to Fererra.) If you have a farm of 200 dispensers, you'll earn 100k per day (again 1.5x more if sold to Fererra.) This makes it a 20-day grind after building the farm.
Another Method is by growing gift plants (which don't regrow) to make 225 per plant or growing Sapplants that sell for 199 per plant and regrow every 6 days after the initial "bloom." This method assumes you have the perk that makes the plant more valuable. The greenhouse can hold 128 plants, which garners 25,472 gold per week. This method earns 100k every 24 days which means this method takes 480 days. There are only 112 days per year.