3 different listings @ Sportsman's Guide and none of the prices are that bad (the first one is Brownell's mags and the last link has a 6-pack option):
Brownells 20 - Rd. Ar - 15 / M16 Military - Spec Mag - 679612, 16 - 20 Rounds at Sportsman's Guide
20-Rd. Ar-15 / M16 Mag - 323004, 16 - 20 Rounds at Sportsman's Guide
Ar-15 / M16 30-Rd. Mag - 180351, 21 - 30 Rounds at Sportsman's Guide
Brownells 20 - Rd. Ar - 15 / M16 Military - Spec Mag - 679612, 16 - 20 Rounds at Sportsman's Guide
20-Rd. Ar-15 / M16 Mag - 323004, 16 - 20 Rounds at Sportsman's Guide
Ar-15 / M16 30-Rd. Mag - 180351, 21 - 30 Rounds at Sportsman's Guide