You might want to add a qualifier to this question (such as, "at the time of their election"), since the wealth of a president is likely to soar after election if they are anything like some UK PMs. A lot of charging for dinner speeches, meetings, etc. That sort of thing to pad out the bank account.

The richest would be Kennedy. His family estate is worth an estimated (inflated) $1billion (yet one report stated only $125million, which was likely his personal interest along with Jacqueline's heiress wealth). The true richest would be Washington at (inflated) $500million.

As for the poorest, I was not able to find an exact answer. Since majority of reports I looked into stated wealth in the form of millions, there were about 10 listed simply as "less than a million" without going into more detail. Wikipedia did state that Truman was the poorest President since 1929 with under a million.

Sources:
Ranking Every U.S. President By Net Worth from Inside Gov
The Net Worth of the U.S. Presidents: From Washington to Obama from The Atlantic
The Net Worth Of The American Presidents: Washington To Obama from 24/7 Wall St

May also be interesting to identify those who may have inherited their wealth and those who were self made prior to ascending to the office.