I bet they are paying well below the equivalent of 1.5% on mileage accounts. Their parent company, Texas Capital Bank, doesn't break out the portion of their deposits that are mileage accounts but their overall cost of checking accounts in the first quarter was 0.29%. For savings deposits it was 0.81%. See link here: http://investors.texascapitalbank.co...943&highlight=

Note that their checking accounts nearly doubled in the first quarter from the prior quarter. I am speculating here, but I wonder if a fair number of those new accounts were mileage accounts. If that's the case, then the avg cost of a mileage account cannot be that high because the rate only went from 0.19% to 0.29% from the 4th quarter of 2009 to the first quarter of 2010.

We cannot make any firm conclusions without knowing the exact proportion of mileage accounts to non-mileage, but it does look like either (1) the number of mileage accounts they have is very small in relation to regular checking accounts, or (2) their cost of checking is well below 1.5%.