I place 1 oz into 3 gallons of water and mix it before filling the remainder of the bucket with water...
One bottle is $12. Each bottle has 22 oz. of shampoo. According to the bottle you can create 25 batches with that 22 oz. Thats about +/-$0.48 per bucket.
I buy my UPGW on Bogo. That makes it +/- $0.24 per bucket. However I use 1 oz per wash so I spend $0.27 per bucket.
If not on bogo and using one ounce per bucket that would make it $0.55 per bucket.