Aside from the math used, there is the conflict of interest between those doing the rating/ stock analysts and their level of involvement with the companies that pay for their services. Recent meltdowns have brought about a wave of honesty as if by magic.

Bottom line real world sales of some "toxic" debt have been running about twentyfive cents on the dollar. You can bet the guys buying this stuff have a very good handle on the valuation, those holding it may try to fluff the numbers to make their books look better.