Modular houses can cost as low as $50/sqft. This Sears house looks like 900-1200 sqft, so 45k to 60k.
http://modularhomeowners.com/how-much-will-my-modular-home-c... https://blog.capterra.com/the-pros-cons-and-cost-of-modular-...
http://modularhomeowners.com/how-much-will-my-modular-home-c... https://blog.capterra.com/the-pros-cons-and-cost-of-modular-...
The reason why I mentioned a Tiny House or THOW is that the comparison is much closer, I think like most countries in the past there were no planning permission or taxes on house building. A THOW too is built by the owner, but it is 200-300 sq ft and not 1200 sq ft.
The website you listed suggests a 50k modular house costs closer to 130k to install once everything is worked through. Do you believe the Sears house cost ~150-200% more in the end? Sure they had to dig wells too but I don't think that is plausible.
https://archive.org/details/SpecificationsAndBillOfMaterials...
It seems like there should be a collection of historical lumber prices somewhere. Just that comparison will be pretty informative about the relative material costs.