This isn’t complete in its cost analysis since I don’t have a price for vanilla, but I don’t think imitation or real vanilla will add much to the final cost.
Basically, this is a mix that is made up ahead of time with the wet ingredients being mixed in at the time of baking. I did the calculations for white flour, but I’ve also used wheat flour with great success. I frequently use half butter and half margarine, rather than all margarine, which would be a bit more expensive.
5 cups sugar 1.15
3 3/4 cups white flour .34
2 cups cocoa powder .96 (this could be lower using the store brand)
To mix, use 4 1/4 cup of mix, plus
3 eggs .39 (could be lower by substituting soy flour/water for 1-2 eggs)
1 cup margarine .25
2 tsp. vanilla (no data)
The mix usually makes two batches of brownies.