Since prices vary, recipes vary, and the coupon queens of the world shake things up quite a bit, here is how I do the math for a 5 gal bucket:
Fels Naptha: $1.12 for a 5.5 oz bar. The recipes says use a 2/3 of a bar. I calculated 4 oz. just to make the math a little easier. The cost of the Fels Naptha per batch is: .80 cents per 5 gal bucket
Washing Soda: $3.24 for a 55 oz. box. This is .06 cents an ounce. I weighed a cup of the washing soda (8 .4oz). I then multiplied the 8 .4 oz. by my per oz. price= .06 X 8.4 oz This comes to .50 cents per 5 gal bucket
Borax (your optical brightener & deodorizer): $3.38 for a 76 oz box. This was .04 cents an oz. A cup of borax weighed 7 oz. Multiply 7 oz. by .04 oz for this recipe of a 5 gal bucket. This totals .28 cents for a 5 gal bucket
My final numbers are .80 + .50 + .28 = $1.58 cents for a five gallon bucket
5 gallons translates to 80 cups but you only use 1/2 cup portions and many people use less with great results (HE washers especially). SO that is 160 half cup portions to a five gallon bucket. At 1.58 for the whole batch, let's see how much per load.....
1.58 divided by 160 = .009 cents a load!
Convenience and cost may cause you to make your own WASHING SODA (I used to buy it at pool supply stores and it was expensive!) Just bake baking soda for 45 to 60 minutes on a cookie sheet at 400 degrees until it changes consistency. Baking soda is sodium bicarbonate and washing soda is sodium carbonate. The baking it causes a molecular change as it loses a carbon atom. If you don't want to do the science experiment and cannot find Arm & hammer washing soda locally, buy it online. It's cheaper to bake n make ;)
Borax is a naturally occuring mineral :Sodium Borate. It is a laundry whitener and deodorizer. You should be able to find it in th laundry isle at WalMart.
Essential oils can be added for increased expense and fragrance. One ounce of oil should be added to your 5 gal bucket. Prices vary there but you should buy it in a dark colored bottle to ensure quality oil.