This will unlock once you have $1,000,000.00 in funds in your pocket at one time. It is not a cumulative total. This will be one of the last achievements you will unlock in the game. Once you hit a point where you have all of the products available for sale just start hoarding money. Only spending money on the necessities until you have the $1,000,000.00. Personally I would save up the $1,000,000.00 and then work on progression towards other achievements like
or
if you do not already have them.
This achievement will unlock after you close your store for the night once you have $1,000,000.00 available to spend.