> As property prices have increased, I doubt that the cost of building the house is even the major cost factor - it's probably mostly property value for the lot.

I'm not so sure. Looking at my property tax assessment my 1/5th my 1400 sq ft house + 400 sq ft detached garage has an accessed value of about 3.5 times that of my 1/5th acre lot.

Accessed values here are based on market values and are reasonably close to what I see when I compare to recent sales on Redfin or Zillow, so it looks like the total is close. And the accessed value of the building is reasonably close to what my insurance company says it would cost to rebuild them.

Checking Zillow for lots for sale, it looks like my assessed value is reasonable.

I'm in a relatively low density area though. As a check I looked at a nearby significant city, Seattle. Comparing lots for sale there to nearby similar lots that have houses on them it looks like it is similar to where I am in some places and very different in others.

It looks like if the lot is zoned for commercial use or for tall buildings the price if very high. I saw one that was something like 0.38 acres for nearly $7 million. But for lots in single family house areas it looked pretty similar to what I'm seeing at my place. The lot is around 1/4th or 1/5th the value of the property.