I was attending a panel discussion last week called VU 2006 hosted by New York University’s Furman Center and the New York Times. One of the phrases that came up repeatedly was affordable housing and the question was asked: “How do you define affordable housing?”
According to The City of San Diego :
The U.S. Department of Housing and Urban Development (HUD) defines “affordable” as housing that costs no more than 30 percent of a household’s monthly income. That means rent and utilities in an apartment or the monthly mortgage payment and housing expenses for a homeowner should be less than 30 percent of a household’s monthly income to be considered affordable.
The median family income of the specific market is used to calculate the affordability.
I think there is the tendency to associate affordable housing with housing for the poor. However, its use today is primarily in reference to the middle class. Worker housing is affordable housing [AHI] 
Cities and towns across the US are losing workers who have become priced out their markets [NYT] . Its a real problem and a hidden future cost of municipalities as they try to replace these workers.
For example, New York City will offer housing subsidies [NYT]  of up to $14,600 to entice new math, science and special education teachers to work in the city’s most challenging schools.
The housing boom has priced out many in the middle class who have been forced to seek other areas for affordable housing [ContraCostaTimes.com] .
This type of gentrification has long-term implications for the haves and the have-nots. Local retail stores, forced to leave because of rising retail rents, are replaced by chain stores reducing the appeal to the area by the buyers who were attracted to the housing in that location in the first place.
I can’t seem to get past the irony here. The very residents that created a community that attracts more residents and development are the ones being displaced.
Thats something the new residents simply cannot afford.