Actually..
Actually the AK47 (or Avtomat Kalashnikova model 1947) is popular for more reasons than it being "cheap."
Yes it it is true that the AK assualt rifle is very inexpensive to produce and buy. However this comes from the fact that it is made from stamped steel and requires few moving parts. In all one can produce as much as 400 times the number of guns than any other variety of automatic rifle.
Popularity also based in "response" or how well the gun performs. For a "buzz" gun (this comes from the sound of the gun set at full auto) the Kalashnikovs are surprising accurate. even though it is chamber in the 5.56 NATO (or .223) the AK makes the most of this miniscule round by grouping shots at .5 inches at 150 yards, the m-16/AR-15/M-4 without extreme custom work can only come close to 2.5 inches. Which brings me to another point, reliability.
The AK is also known for extreme durability. The American equivalent, the Colt M-16 (and similar copies) is by far the worst in this category. Much to the dismay of our officers, during Viet Nam (and even today's current Iraq skirmish) our soldiers had a tendancy of using the Viet Kong's AKs. This is not to say that our own arms are inferior, but the AK's reliability (i.e. it won't jam every other shot) is unmatched.
I've shot a number of both styles of guns (AR vs AK) and will gladly choose a custom built DPMS m-16 or any AK copy, but for off the production line, I'll stick with the Ak.
~Joker a.k.a the Original Armorer.