US real estate valued about $18T according to Case Schiller
In 2007, it's about $22.5T. Assume 20% drop since then, that's about $18T.
http://www2.standardandpoors.com/spf/pdf/index/SP_CS_Home_Price_Indices_Factsheet.pdf
However, this refers to real estate held by households and nonprofit organizations. Don't know what's the split between these two.