How much is the average mortgage amount in the us?

As of the end of the fourth quarter of 2009, there was approximately $14.3 trillion in outstanding mortgage debt outstanding. Of the $14.3 trillion, $2.5 represented non-residential debt, leaving $11.8 trillion due to residential owners.
At the end of 2009, there were approximately 129.9 million housing units in the United States of which 66.2% were owned by the people living in them. Of this 86.0 million housing units, approximately 20% own their homes "free and clear" (except, of course, for property taxes), suggesting that there are approximately 68.8 million housing units with debt underlying the ownership.

The average mortgage balance, then, is just short of $172,000 (and, evidently 10% of the population have mortgages outstanding of over $250,000).