I don't think you can get any cheaper than a single layer of boards nailed to the purlins. Not a great roof, but it gets you a few years in the dry. I don't know if they were shiplap or feather-edged or exactly how they kept the wet out.
I guess the shingles were pricey by comparison. There is certainly more labor in the install and more work in the making of them.
I'm not an expert on this, but it is interesting.
