[vericiab]

By how much and why the cost of labor has changed over time is an interesting question. I'd assume that the bulk of the cost of labor is wages, but there are some other direct and indirect costs that might contribute in addition to wage related factors.

Carrying workers' compensation insurance was not the norm at the turn of the 20th century; injured workers could instead sue their employer for negligence but usually lost. Legal challenges to state legislation that made participation in a workers' compensation program mandatory weren't resolved until the Supreme Court's 1917 ruling on N.Y. Central R.R. Co. v. White. In the modern day, Texas is the only state where workers' compensation insurance isn't mandatory.

In a similar vein, workplace safety standards have seen a lot of change. With the exception of a few states that passed legislation sooner, workplace safety was largely unregulated until the Occupational Safety and Health Act of 1970 established OSHA.

I know less of the specifics but consumer protection laws have also changed. Many states now impose statutory warranties on new construction that cover construction defects for a period of time. It seems plausible that builders had less liability for such defects in 1914.

Construction equipment and the cost of running and maintaining it is another thing that I would expect gets rolled into the cost of labor. I'd assume that equipment costs are usually offset by increased efficiency (paying wages for less time), so it wouldn't increase the cost of labor, but it would be interesting to see data on that.

As far as wage-related factors, while blue collar wages may not have changed in general there are some exceptions. Based on the sources you provided, plumbers in 1913 San Francisco worked 44 hrs/wk and made $0.75 per hour for an annual wage of $1716 (or $44.9K in 2020 dollars). The median plumber in 2020 San Francisco makes $110.9K/yr. (Since SF might be an outlier, in St Louis in 1913 plumbers made $1515 annually, equivalent to $39.6K in 2020, and had a median annual wage of $80.6K in 2020.)

It's also possible the number and types of workers required to build a home has changed. Constructing a new home might now require more "specialists" (like plumbers) who command higher wages to do work that was either not done at all or was previously done by general laborers paid lower wages. It would be interesting to see data on that as well.