Because AK47 is one of cheapest weapons to produce. And since there are so much of those weapons, price is low. Besides it's not like they use new or original ones, they may as well have alternative versions of those with even cheaper parts.
The AK47s that are floating around the blackmarket today are all from the Russians. Ever since the demise of the soviet union, warehouses of AK47s were raided by or sold to illegal arms dealers that in-turn armed organizations like Al Qaeda, African warlords, and even modern pirates. They don't produce these weapons, all of them are stolen from derelict military stockpiles and then resold to the highest bidder.
Ak47s sold in bulk are relatively cheap.
Relative to what the United States' military pays for their weapons yes, but by no means cheap.