RE: Cheapest round?
If you want a long gun for home defense, nothing beats a good 'ole 12 gauge pump gun. Simple, reliable, and loaded with 00 buck, there isn't a man on Earth that will walk away from 9 pellets of buckshot to the chest from 10 feet away. It'll also penetrate less structural material than a 9mm pistol. An AR-15 with frangiable bullets is a good second choice.
I remember reading about an FBI study on the penetration of various bullets through common building materials a while ago. At the time they were toying with the idea of switching from the MP-5 9mm submachinegun to the M4 carbine for their tactical units after the Hollyweird bank robbery debacle (the two bad guys covered head-to-toe with two layers of ballistic armor, with real-deal AK-47's, that held their own against several dozen LAPD and FBI armed only with shotguns and their sidearms until the cops eventually took them out with rifles borrowed from a convieniently located FFL dealer). But they were worried about the rifle overpenetrating or passing through (several) walls and killing innocents on the other side.
Anyway, they shot the 5.56mm NATO and 9mm Parabellum (and some others pistol calibers too, if memory serves), through various types of walls common to residential, commercial and industrial buildings, and found that the 5.56 NATO M4 penetrated through fewer residential-type walls (2x4 studs, drywall) than all the pistol bullets, and also in geletin tests where the round was fired through a wall at various angles and then into ballistic gellatin on the other side, the 5.56 displayed far less penetration and potential lethality than the pistol bullets. At the same time the 5.56 showed far greater projected lethality than the pistol cartridges on direct contact wounds, and was able to penetrate 2 layers of class IIIa light body armor without a trauma plate (similar to what the Hollyweird bad guys were sporting), and one layer with a trauma plate.
Just food for thought... Have a Merry and Blessed Christ-mass!
Mike