I don't know what the saiga's go for now, but I used to have one in 7.62x39 and it cost like $230 from my local gun shop, the shotguns ran about the same price. I haven't looked at them lately, but I doubt they went up hudreds of dollars. Johnson's Sporting goods in Adrian MI.
Um, yeah, I've tried a .410, here is the one I own. It's a Winchester 9410.
I have never seen a 410 slug penetrate any kind of steel. You are talking a fairly light soft lead bullet traveling at 1500-1800 fps. A 9mm will fire a full metal jacket hard bullet from 115 grns to 150 grns at velocities around 1000-1500 fps, and using a carbine adds about 10-15% to that. Not to mention much better bullet choices. And the .40 cal is even better than that. I have put them clean through car doors out of a pistol. Not to mention an AR in 9mm or .40 cal is probably going to much more accurate than a smooth bore .410.
There might be better .410 loads, but your not going to find them easily around here. Most of what you will see are going to be 90-100 grn soft lead bullets at around 1600 fps.
Make one of those babies in a 20 ga and you might have something, especially with a rifled barrel. I beleive there would actually be a market for that around here. I'll stick with a 12 ga for home defense though.
Paul