I spent way too long trying to get this on my own crappy farm, but ended up popping it really quickly on another player's online farm, while going for the unrelated "Many Hands make light work" achievement:
Try to find a high level farm online (using Find Farm from the main menu) with crops, trees animals and ponds as far as the eye can see, grab a full tank of petrol for your mower and work through as much as possible.
Try to target the most exotic plants and animals for the highest amounts of cash. It took me approximately 20-30 minutes before this achievement popped. Hope you have as much luck as I did!