Don't know how many snakes you are feeding, but 1 female rat will produce every 21-24 days, if you leave her with the male at all times. You can expect 5-20 pups everytime, but consider that you need them a certain size, and they will all be the same size at the same time. You will need to freeze at the size you need.
In all reality, you are not going to save money by breeding 1 female rat.
Find a breeder nearby, and buy direct, save your money, and time...