The percentages seem more informative to me, since they are meaningful across different locations. If you don't know what 40% of a home price is in your area, do some research.

I believe that they are not meaningful across locations. Things like solar arrays and bathtubs have a similar cost around the country, but house prices can vary wildly.

This is plausible, but would imply that improvements like these would be more common in lower-house-cost areas, since they'd make a "bigger" difference. That doesn't seem to be the case.