But relatively speaking it was, yes it's not cardboard cheap but it wasn't that outlandishly expensive either, labour was cheap, materials were cheap and the actual process of assembly although time consuming isn't high skill especially in an era when everything is hand-made. When coupled with elements like second hand, chainmail wasn't that luxury of an item. Maybe in the early middle ages when blacksmithing was a bit rarer but still.

Note that throughout the middle ages, majority of fighters were at minimum middleclass, not peasants, furthermore the act of building equipment was done over time and not just spontaneously, so while maybe it would be very expensive to a peasant farmer, to those of higher status, like the servant of a low ranking noble, or a yeoman it would have been relatively affordable.