Well, they get paid ridiculous sums to speak at events, but really they're just being paid for an hour or two of their time. (And a favor of some sort). They're among the most well-connected and powerful people in their political parties even after leaving office, and paying them to do things after they leave office is perfectly legal.

This is a little different than Kanye or George Foreman, who are paid mainly based on their brand value. At this point, stamping a mediocre shoe with Kanye's name is the same as stamping a shoe with a Nike swoosh, except the multiplier is higher.