I dont know if you'll find anything cheaper than the $1/gallon sale at the big box pet stores. The only place I've seen people by from in bulk is glasscages; but I'm doubtful that you'll get any cheaper than $1/gallon.
From looking at the list prices that retail stores buy at the price they sell at is basically what they pay for the tank. Stores make their cash on livestock and decorations for aquariums not on the sale of the tank itself. For the most part the price you pay for a small tank at a retail store is the lowest price anyone (person, or business) can buy it for. That is the wholesale price.