There is no such thing as a proper value for an asset.

There are a number of values. Market value which was brought in to deal with Enron but abandoned as a result of the crisis, as it would have meant that the banks were insolvent. Then there are think of a number as used by the music industry. There is average value. Also values will change over time. The problem is that the values for many mortgages that the banks have used do not reflect the ability of the borrowers to repay them. They are grossly over valued.