Very rough calculation to follow.
Let's assume a worst case 33% drop in prices. Based on the CAAMP survey, roughly 20% of all mortgages have between 20% and 40% equity. (20% is of course the bottom end as anything less is required to be insured and I'm using 40% as the top end to account for transaction costs, which the bank must assume to foreclose and offload the house.) Assuming valuations are flat across equity that's $200 billion in mortgages. I expect that's a little low, given that recent purchases are more expensive and are more likely to have less equity and as well due to HELOCs some houses with less than 20% equity are uninsured too, but let's use that number for now. Roughly half of all mortgages are insured. That leaves us with $100 billion in uninsured mortgages in this borderline equity range.
Let's assume a horrific worst case of 40% of these uninsured mortgages going bust. The banks would face about a 14% loss on average for a total loss of $5.7 billion. That's about 3 quarters worth of profit for the big banks combined. Assuming their profits hold steady, that is.