RE: Cheapest round?
For pure self-defense, like a couple guys already said, just load up your Mossberg with buckshot. My home defense weapon is my Winchester 1200 pump loaded with #4 buckshot- I feel very confident with that by my side.
However, if you want to get a new gun that is cheap and shoots cheap ammo, I'd suggest an SKS. I have one I bought for $120 (this was a long time ago, I don't think you can find them that cheap anymore)- shoots the 7.62x39 ammo that is very cheap (at the local gunstore they sell 25 rounds for $3 on sale). It could serve as a self-defense weapon as well. Only downside is that it's not very accurate- but it is accurate enough for plinking around for fun. Mine's accurate enought to shoot pop cans at 50 yards and milk jugs at 100.