I am not sure who is the lowest, but the way to find out is post the lowest I have seen and let someone better it.
Kiesler's Wholesale 1-888-543-7537 In Shotgun News:
Complete Bushmaster Lower Receiver Assembly
Complete flat top 16" rifle (Signature -X1A, this thing looks weird... has a full length aluminum barrel shroud)
Flat top receiver only $89
Stripped Lower only $124