I seem to remember that they were charging the Secret Service rent for space on the grounds of their Chappaqua estate.
Pretty sure they got their mortgage covered.
Not just that, but I heard that neighborhood is a nightmare now, with checkpoints that monitor every one who lives in the neighborhood and records all their movements in and out of their own home.
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.