This is the most grindy achievement in this game, however I have found a very efficient idle method to accomplish it.
This achievement will take 10-20+ hours
-Find or make the most valuable plant you can find. The most expensive plant I could find was only around 9,000 coins, so if you can get one that is worth more than that, you will be able to do this way faster, I would suggest exploring the final few tutorial levels as they seem like they could have valuable plants, I tried making my own through DNA editing, but I'm not sure which genes directly influence value.
[If you can't find a plant worth more than 9,000 coins, load up the first tutorial level 'Controls' and fast forward until the tree on the left has developed fruits (the tree will fill up in increments, wait until the 5th time fruits appear for max value)]
-Once you are ready, hover hover the plant and click
then
to sell the entire plant
-Now move your cursor to the top right of the screen and buy back the sold plant with
-Now click
again to place it back in the selling slot
-Now repeat 111,111 more times
Buying back and reselling plants adds to your total sold value, ideally you would use a turbo controller to do this, a 5ms delay would be perfect, anything more and you are adding hours to the time it will take.
Notes:
-You do not need to save your progress, it is done automatically, when you are ready for a break, simply press pause, then quit the game through the xbox menu.