(Wealth is relatively easy to measure: Who's poorest -- that's a tougher thing to gauge. The following presidents are in the bottom tier. But we won't try to rank them in order.)
It's well known that before he went into politics, Truman was a haberdasher, but his men's clothing store nearly went bankrupt. His 18 years in Washington didn't net him a lot of money either, but he saved carefully and was able to live comfortably after he left office. He and his wife, Bess, were the two first recipients of Medicare.