From what I recall, that was about $10k/month. Just enough to cover the mortgage, as you pointed out.
Billy gets a Federal pension for being President. He may also collect something from Arkansas for being governor. Hillary would also collect a federal pension for being a US Senator and Secretary of State. Those probably add up to $500K. Not sure where the other $400-500 comes from.
I think he was gov and attorney general, more years for pension payout.