plugger2The real problem is that the mortgage originators...

plugger2

The real problem is that the mortgage originators (whether banks or other financial vehicles) have not been keeping these loans 'in house' as was previously the practice. When kept 'in house' your figures are somewhere near a reasonable assumption.
Unfortunately motrgages of all type (sub-prime, prime jumbo, commercial etc) have been bundled & flogged off to 3rd party "investors" like hedge funds etc who typically leverage up to 10x or higher into the face value of the package.
So a 10% drop in face value of the traunche represents a 100% loss of capital if they are leveraged by 10x.
It isn't hard to see that those sorts of losses (actual or newly feared) has a dramatic effect on the amount of capital available for both new & refinancing.
It is particularly so for refinancing (honeymoon or teaser loans) when the value of the property to be refinanced is now lower than the amount that needs to be refinanced.

In short, it is not so much at the "coal face" of the sub-prime problem ie the actual home owner, that the economy threatening problem lies, it is with the holders of the highly leveraged pooled paper that has those homes (of decreasing value) as tenuous security