2 T. molasses
Mix together with a fork. A food processor would work well, but I don't have one of those. I actually just tossed it all in my mixer with a whisk attachment.
Several recipes I saw said to use 1 Tablespoon of molasses for light brown sugar and 2 Tablespoons of molasses for dark. I used 2 Tablespoons of molasses and about 1 1/2 cups white sugar to get a beautiful golden brown sugar.
I don't know that this would be cheaper than brown sugar, especially if it is on sale, but it was certainly a convenient recipe to have on hand the day I needed brown sugar and couldn't get to the store.