I agree...cost is a bit misleading; and I think probably not a a deciding factor for me. I have both a .22 LR and a .22 HMR. The .22 cal. ammo can often be bought on special in boxes of about 500 rounds or so, they store easily and will last forever if you keep them dry. Buy 500 rounds every payday for a few months and you will soon have quite a stockpile. You can even buy the subsonic (not as loud) .22 shorts that are much better than an air rifle. Not all rifles will handle the short, you would need to check that out. Do some online research on the .22 LR and you will find that if you can hit what you shoot at, they can be effective way beyond 100 yards. The .22 HMR is an awesome round; not as good as a .30/30 or a .223, but I bought it just because I wanted it!
I also have a .17 cal air rifle. It's ok for target shooting and will dispatch a gopher at 20 yards if you can hit it. Some survivalist info recommends a .22 cal break-barrell air gun with as much f/s as you can get. It can be used in a pinch to kill squirrels and rabbits at close range, and perhaps is a bit quieter in an urban setting (hope it never gets to that). I know the .22 Gamo with a scope will easily kill a squirrel or rabbit at close range.
Probably more than you ever wanted to know, but the bottom line is if you want something effective, go with the .22 LR, and if you go with the air rifle, go with the .22 cal, high velocity and don't expect much from it beyond about 25 or 30 yards.