Since 1958 with the Former Presidents Act [1] the Presidency guarantees you'll live very comfortably for the rest of your life with a lifetime pension (and even a small pension for your wife), funding for an office/staff, lifetime secret service protection, funded travel, and more. It was passed precisely because of the scenario you describe playing out with Truman who was rather broke, and ran into financial difficulties after leaving office.

[1] - https://en.wikipedia.org/wiki/Former_Presidents_Act