The Type 80 was a Chinese copy of the Soviet PK machine gun, with minor differences.

$2150 USD open market price 2012

the AK-47 is by far the most used machine-gun to date, it is estimated that 1 billion people have been killed with this weapon in armed conflicts ranging from Africa to Eastern Europe to the Middle East to Central Asia to Southeast Asia…

Except that the AK-47 is not a machine gun, if you want to get technical - it's an assault rifle, whereas a machine gun is a support weapon, usually crew served. In which case, it would probably be the PK machine gun, followed closely by the FN MAG.

AEK-999 (Machine gun - 7.62 x 54 mm)

AO-9 (Machine Gun - 23 x 115 mm)

AO-18 (Gatling Machine Gun - 30 mm)

DP-27 / DP-28 (Light machine gun - 7.62x54mmR) * ** DPM (Light machine gun - 7.62x54 mmR) ** DA (Light machine gun - 7.62x54mmR) ** DT / DTM (Light machine gun - 7.62x54mmR) ** RP-46 (Light machine gun - 7.62x54mmR) * DShK ** DShK 1938 (Heavy machine gun - 12.7x107mm) ** DShK 1938/1946 / DShKM (Heavy Machine Gun - 12.7x107mm) Kord (Heavy Machine Gun - 12.7 x 107 mm)

KPVT (Tank Heavy Machine Gun)

Maxim M1910 (Medium machine gun - 7.62x54mmR)

NSV (Heavy Machine Gun - 12.7x108mm)

* PK (GPMG - 7.62 x 54 mm R) ** PKM (GPMG - 7.62 x 54 mm R) ** PKSMN (GPMG - 7.62 x 54 mm R) ** PKT (GPMG - 7.62 x 54 mm R) Pecheneg (Machine gun - 7.62 x 54 mm) SG-43 Gorunov (Medium machine gun - 7.62 x 54 mm R)

TKB-523 (Light Machine Gun - 7.62 x 39 mm)

6P62 (Heavy Machine Gun - 12.7 x 107 mm)

50 dollars