Borrowers will prioritise paying their mortgage more than anything else, so even in a downturn mortgage payments will keep flowing to the banks – but earning a 2% net interest margin doesn't amount to much if you're writing off 100% of a bad loan. All of the major banks are down more than 1.25% this morning, the ANZ share price is down around 2%. Foolish takeaway Commonwealth Bank shares have dropped 4% since the pre-open price on Tuesday. The CBA share price is still up compared to where it was just a month ago – so it doesn't strike me as a bargain buy today, although a few more days of declines may make it more attractive.