You rarely see a truly great scatterplot, but this chart from Trulia compares the median asking price per square foot in different housing markets against the pace at which the supply of houses in the market has expanded:
You see that there are all kinds of affordable cities in the United States. There are plenty of low-demand cities, especially in the midwest, where the housing stock has grown slowly and prices are low. But there are also plenty of cheap, fast-growing cities in the sunbelt where the housing stock is growing rapidly and keeping things affordable.
And then there are the expensive cities. The places where house-sellers are asking for over $200 per square foot. All of them are cities where the housing stock is growing slowly, even though these are the places where it would be most profitable to build. That's because these cities tend to have geographical constraints that prevent further sprawl, and have adopted zoning codes that make it difficult to add more housing by building more densely.