Dutch Mill

Cost: 12RC (one of these already on the farm at the start of the game)

Wheat flour:
Sell Price: 62 coins
Products used: Wheat
Profit: 19 coins (30.6%)

Add to Baker to get Wheat Bread

Used in Cake Machine for All Cakes

Oat Flour:
Sell Price: 56 coins
Products used: Oat
Profit: 17 coins (30.4%)

Add to Baker to get Oat Bread

Rye Flour:
Sell Price: 87 coins
Products used: Rye
Profit: 37 coins (42.5%)

Add to Baker to get Black Bread

Rice Flour:
Sell Price: 136 coins
Products used: Rice
Profit: 41 coins (30.1%)

(The profit I give is based on what it costs to plant the initial crop used. If the items used to create a product are not from a crop, their cost isn't factored into the profit. It's as though I'm assuming the cost is incomparable (RC to coins) or that the item is "paid off").