Yes, but can you build the Sears house for 35k today without materials substitutions. I don't think you can, prices of commodities were probably lower then, which is not in line with received wisdom.

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.