Portland was a city
with very affordable housing.
Not now -- here's why

From the New York Federal Reserve:
In The Impact of Building Restrictions on Housing Affordability the authors argue that in the most of the country housing prices are very close to construction costs.  Where prices are high relative to construction costs, economic analysis shows that the major culprit is restrictive zoning.

They also point out that many popular policies, like smart growth policies to increase density, affordable housing mandates, and subsidies, don't do much to help. Attacking zoning and other regulations on housing is the best way to make homes more affordable. Download this report from the New York Federal Reserve web site.

From Reason Magazine:
In Repairing the Ladder: Toward a New Housing Policy Paradigm, Harvard University's Howard Husock lays out a new approach to affordable housing policy.  Download this report from Reason Magazine

More on Housing Policy:
UrbanFutures.org has a wealth of information on housing policy