The lot is always cheaper, as long as the land is cheap. And in most of the US, even land that isn't all that cheap is often best left as a parking lot, economically: You can easily speculate with a parking lot with minimal investment, as the taxes for the empty lot are often low. See all the midwestern cities whose downtowns are 30-40% surface parking.

There are all kinds of bad externalities caused by seas of asphalt that is unused 95% of the time, but few countries are all that interested in using any mechanism to make the property owner pay for them.