I don’t think there is such a thing as an inexpensive AR.
This DPMS is one of the lower priced ones I’ve seen.
http://www.dpmsinc.com/store/products/?prod=5295
Ammo doesn’t seem to be that much of a problem right now, but in most cases is about 20 to 30 percent higher than a year ago.
You can get a cheaper Saiga .223 that will shoot less expensive steel case ammo.
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I just got one NIB for $350.00, any decent AR costs 3-4 times that.