You borrow £100 from your bank, as do 100 other people, totalling £10,000. The bank packages them up into 10 £1,000 packages and sells them on to other banks with the return being made on the mortgage repayments you make.
So bank A sold to bank B. Now you default on your pament because you don't have enough money, the value of your house has dropped 25% ( as in some areas in the US). Bank B writes off 30% of its purchase, from £1,000 to £700. This is due to lack of repayment of principal and servicing of the debt.
The 30% is an estimate because banks have bought packages of mortgages so they take into account default rates, property values etc. Some of the banks might eventually find that they have written off too much, but valuation of an investment is based on the current market value. If there is no current market value because no-one wants to trade, how do you value it? If you go back to a quant model then your valuation and ultimately your writedowns are going to be based on key assumptions.
To aardvark, the credit derivative market might be the size that you are claiming, but I don't know how you can make it out to all be mortgages and potential losses being so large as its not all junk debt? There is a fixed size of the US housing market which is taking a hammering because of the no principal, low repayment mortgages which caused the crunch. With the market assuming that a majority of the writedowns now done, you're standing out on a bit of a limb saying that you think future writedowns could be 4 - 20 times larger? If you are saying $5 trillion is a conservative estimate, which is 20 times larger than the current writedowns, then keeping everything proportionally the same, UBSs' writedown should increase by 20 times. As they have written off almost $40bn, I am a little sceptical about your 'conservative' estimate that they will write off another $800bn (20 * 40bn). By my estimate, their current total assets as at December 31st 2007 were roughly CHF 2,272 billion, or $2,163 billion. That would be quite the chunk of their total assets. Even if we halved this conservative estimate to 10times, so $400bn. I doubt that UBS had 20% of its total assets in CDO/MBSs. Especially as I would have thought bank regulations would not allow this.
And as regards the banks not wanting to price their assets, you can't price something that doesn't have a market price. Banks have to price their assets to a market value when they present their annual statements, as UBS did on 31st December. This 'banks don't want to so don't' idea doesn't really work.
my 2 cents...