RE: Cheapest round?
Round wise? 9mm, 40 S&W, 45 ACP, 7.62X39, 5.56, 12 Gauge, 20 Gauge, and about any other round you could want. My favorite self defense rounds is my Mossberg 835 loaded with 3.5" 2 1/4oz no.5 Turkey Loads. They dont penetrate exterior walls but makes a major mess of whatever is hit with it within 10 yards. I would much rather hit someone with a couple hundred small pellets then 1 slug or 8 pellets of buckshot.