It's kind of a strange calculation to make, because waged labor and alienation weren't common features of medieval society. Peasants lived in villages that collectively possessed all the raw materials, tools, and skilled labor for producing the shirt or anything else one might need for day-to-day life. They were small enough in both population and economy that family ties and other social obligations meant that everybody owed everybody something just in the course of living and working there. Maybe this would be a valid comparison if everyone lived and worked in a self-contained farm-to-body everything-factory coöp that clothed its own employee-member-owners as a priority to sales for profit.
I largely agree. IMO the $ price tag doesn't translate well, because time of manufacture was the real cost, and likely the limiting factor on production. But then there was also limited demand. I doubt more money would go to a fourth shirt; there wasn't much class mobility, and a the benefit of a fourth shirt was likely less than money spent in another sheep, a hammer, etc. You lived the life you lived. Why buy a shirt you didn't need? Shirts serve a different function today.