Possible changes suggested for Fannie Mae and Freddie Mac could lead to borrowers paying an extra $400 a month in mortgage payments, according to a new analysis from Zillow. If the 30-year fixed-rate mortgage were to be done away with, Zillow said future mortgage borrowers would get loans with shorter terms and higher interest rates. For example, without the popular 30-year fixed-rate mortgage, the typical buyer would pay an additional $390 each month on the median-priced home for a 15-year fixed-rate mortgage. Moreover, the conforming market would move closer to the jumbo sector. Zillow noted that a 30-year non-conforming loan would cost borrowers about $20 more per month than they now pay.