The OP asked about CHEAPEST ammo..........
The way to get the cheapest ammo is to make it yourself. Easy when you know how. Classes are available.
You can make 9mm ammo for about 1/2 of what you can buy it on sale for. That gives you twice the shooting for the same money.
Was at my club the other day, and another fellow was there shooting. I asked him if he did a lot of shooting. He replied that he didn't, because it cost so much. He said that he bought 50 round box a couple of times a week. It was the cheap berdan primed, steel cased stuff. He complained that it didn't shoot well, and he had a lot of failure to fire.
I tossed him three boxes of 9mm reloads. I suggested that he shoot them up, and really have fun with them. He sheepishly asked me how much he owed me. While telling him how cheap they were to make, I fired my gun and nailed the heck out of a paper plate at 75 feet away, pretty much rapid fire. When I mentioned that the reloads are better than the ability of the shooter to shoot them, he was hooked. I would never charge anyone for my reloads, and don't often hand any out. He shot the three boxes in short order, and said that he improved his shooting by being able to shoot more (even the three extra boxes helped a LOT). He also said that it was the most fun shooting that he had ever experienced. Because ammo is so expensive at the stores, he always shoots slowly, savoring every moment. By having so much ammo to shoot, it was much more fun, and helped him shoot better at the same time.
So, if you want better ammo, and cheaper ammo, make your own...............
I'll step down from the soap box now......................