After collecting all the coins, a notification will appear, saying that they have no value. This however, is not true. To obtain the reward, you have to go to a mysterious well, located in the bottom left corner of the map, near field no. 40 (coordinates: [229,1714]).
When approaching the well, another notification will appear, saying that you receive a reward. Your warehouse and silos will be filled to the brim. Depending on the level of difficulty and the prices in shops, you will be able to sell the supplies for at least: 2 millions (easy), 1 million (normal), 0,5 million (hard). Sell the supplies where the prices is high and you will earn a fortune. Congratulations!