They're pretty fun to pop off at the range but they are completely impractical to use for anything else. Anything that an AR pistol can do a normal handgun can do more effectively for the average shooter. They will tear you up when firing them though without that cumbersome brace support thing. I saw someone rapid firing an AR pistol at the range once without the brace and he almost dropped the damn thing. I moved pretty far away from that guy lol.
The AR is the most popular rifle in the US so it is not surprising that prices are lower due to the popularity and production. It is also not surprising that they are used in shootings due to being the most popular rifle. Something like 3 million of them in the US and at least 10 times that in 30 round magazines.
My buddy showed me an ad for an AR kit (everything minus a stripped lower) for $279 delivered to your door. My barrels cost more than that. But if you are going to feed it surplus 2 MOA ammo, no sense breaking the bank on accurate parts and optics.
O there are more than that it was 5.6 million from 2000-2014. Which doesn't include any other semi auto rifle, just AR 15s. So not even AR 10s. http://knowledgeglue.com/just-how-many-ar15s-are-out-there/