Probably from an unused root meaning to enumerate; an assessment (as based upon a census) -- tribute.
H4371. mekes
מֶ֫כֶס noun masculine computation, proportion to be paid, tax (Palmyrene מכסא ReckendZMG 1885, 379 ff.; Late Hebrew מֶכֶס, Aramaic מִכְסָא, tax (hence Arabic as loan-word Frä283); Assyrian miksu, DlHWB 407 (√ מכס); Late Hebrew מוֺכֵס tax-collector; so Assyrian makkasu, compare Arabic ) — Numbers 31:28 מ ׳זַהֲרֵמֹתָ֫ ׳לי and thou shalt separate a tax (duty levied on the spoil) for ׳י, 31:37; 31:38; 31:39; 31:40; 31:41 (all P).