Bought most of them at the BX here, but their price went up nearly $1 in January. Prior to that they were $12.46, IIRC with no shipping or sales tax.
Online I buy them from DSG Arms almost exclusively. With a mil discount $11.51 for black and FDE and $11.21 for FG and OD. I don't remember what shipping ran but I usually place a pretty big order with them when I buy so it works out to a tiny amount per mag.
I think I have over 50 of them now.