UPDATE: THIS GLITCH HAS BEEN PATCHED, HOWEVER YOU MIGHT BE ABLE TO UNINSTALL THE PATCH, AND DO THE ACHIEVEMENT OFFLINE.
Since most people are going to do this on a second playthrough, I thought i'll inform everyone of an exploit that makes this achievement a little under and hour to obtain. On the 27 of Fall the Spirit's Eve event will occur at 10pm. At this event you obtain the golden pumpkin at the end of a simple maze, which can be sold for 2500g. However there is a way to duplicate the pumpkin.
*to speed to the 27th of Fall you can just sleep away the days*
1. Make sure you have a full inventory, this will not work without it.
2. When you take the pumpkin from the chest, drop it on the ground a leave the menu.
3. Open the chest again and drop another, repeat this 10 times.
4. Once you have dropped 10, open the chest to access your inventory (during the event you can't access it normally) and delete one item. When this happens the game will pick up the 10 pumpkins on the floor and glitch giving you 512 pumpkins. DO NOT TAKE ANOTHER PUMPKIN.
Doing this will take away pumpkins, for some unknown reason.
5. Now leave the festival and sell the pumpkins, granting you 1.2 million, NOTE: this will also give you those money achievements if you didn't have them already.
6. Now the next day got to the JojaMart and purchase the membership for Morris and wait a day.
7. Now the warehouse will be built and each day you will have to buy one project from Morris.
8. Once you've bought all 5 upgrades go the warehouse for a small cutscene and boom, achievement unlocked!
Also for those wanted to do the exploit in there main playthroughs, the max amount of money you can get is ~3mil. To get that much you need to take out 31 pumpkins, and you will have to trash 3 items instead of one. I wouldn't recommend doing the exploit if you are enjoying the game because it allows you to get everything really early. This is good because it can cut your time in down for a lot of achievements, but it removes a lot of the challenge.