I am not sure where you got the $16 proce tag for a marine Corps shirt from. If it was a site geared to selling to the public, then the shirt may be "Mil-spec" but not US Military. Most "Mil-spec" shirts that are sold to the public are made in Central America and are NOT used by the US Military - and are cheap.
If the site was geared toward selling to military personel, I would guess that government subsidies come into play.
Regardless, Military uniforms are produced in substantially greater numbers than BSA uniforms which can also account for the cheaper price.