OK, I figured it out.
My per 1 liter of D-72 costs me $2.06 in raw ingredients, excluding shipping. If I were to make a gallon of it, it will cost me $7.83 which will be higher than Kodak's bagged version at $5.79. My high cost is probably because I'm buying chemicals via the most expensive route, retail photography stores. I'm sure if you buy by the barrels, the cost will be substantially less.