Arrrgh! I've been trying to keep off this thread... I had a 24 .22/410, have a deluxe 24 .223/20, and wish I had purchased a couple of 24C camper models in .22/20 years ago when they were inexpensive and widely available...
I grew up with a single shot 20ga and harvested a truckload of game with it. It's most versitile feature was/is a factory option PolyChoke - bunnies in heavy cover were dialed wide open, of course. I was never handicapped by its gauge or lack of a magazine. Very lightweight and swings like a dream - nice balance.
But I've been using 12 gauges for the most part for about 25 years now. There is a potential difference if family protection against large critters is the reason for lugging it around - I greatly prefer 12 gauge for that. Hunting-wise I shoot neither better nor worse. Recoil is worse in the 20 ga single shot, by the way - it is far lighter than my 870 Wingmaster, side-by-side, or Berreta auto 12. But not so it bothers me a bit - I grew up with it.
I don't see any point in arguing the hunting merits of 12 vs 20 gauge - the gun and the gunner make the difference for the most part.
As for cost - Buy a case at a time and it's the same price (which means the 20 ga is more expensive in terms of cost per ounce of shot and grain of powder, but a case of shells is the same count in either gauge). At least, that's my experience here - and a case of shot gun shells is not all that many rounds nor is it all that expensive. Cheaper by the case at a gun store than by the bargain box price at Wally world - YMMV, of course, but check it out - buy a case after the last hunting season in the spring (turkey) and before they re-stock for the fall. Pay thru the nose in the fall, of course...
OK, I got that out of my system now...
Tom