Yeah, unfortunately my main source (Haythornthwaite) doesn't mention infantry buff coat prices in his section on equipment, and most of the plates with infantry depict them in what appears to be a buff coat and breast-and-back, which means the coat doesn't get described on its own.On the other hand, I've yet to see a decent descriptions of the type of jacks/coats an NMA infantryman was wearing, it's often described as a "buff coat", too, but I doubt that it would have much in common with the posh cavalry pieces. Maybe more the usual leather jerkin to prevent chafing and thus not armor per se.
Note that by this time we're almost exclusively in 17th century England, which is a bit outside of my area of interest (and not applicable to most fantasy settings, too).
But leather and linen was mixed before, too. The aforementioned King's ordinance recommended "stag skin", if I remember correctly.