Subprime losses could reach USD 400 billion

I saw a few estimates that put the subprime losses as USD 400 billion.

Here is my guess about how this number is arrived:
number of houses in USA - 100 million
proportion that are subprime, say 20%
number of subprime mortgages: 20 million
proportion in default: 50%
defaulted mortgages: 10 million
average loss per default: USD 40,000
total subprime losses: USD 400 million

My guess could be wrong. For example, it may be fewer mortgages with a larger average loss, or more mortgages with a smaller average loss.

The large financial institutions have reported losses under subprime mortges in the magnitude of USD 10 million. If this is the average loss, a total of 40 financial institutions would make up the total of USD 400 billion.