The easiest way of doing earn this is to collect the 100 coins scattered around the map as after you've found them and placed them in a well in the bottom left corner of the map, all of your silos are filled giving you maximum amounts of each produce which can then be sold, easily taking you past the $1M mark.
I've created a video showing this in full, locating the 10 quickest coins to collect (5 at the main farm area and 5 in the town at the top of the map with the lighthouse) which then places markers for all others on the map.