I use to hitchhike all over the country back in the day and I still hitch around from one trailhead to another, but never very far anymore. Back then I never had any money, but even now I don't think people ever expect money or would want you to offer it. Maybe buy them a beer or soda or a sandwich, something in trade essentially, yes, but not money. I'm not saying I wouldn't be willing to pay them, it would certainly be worth $20 to me, but I think people would prefer just to give it to you and get your profound gratitude in exchange. Well, and also they usually like for you to listen to their stories, laugh at their jokes, etc. That's just good manners. I try to be the most polite of audiences possible.
Now, one exception to the money rule would be if I were needing a ride that would require them driving significantly out of their way. In that case it would quite reasonble (and in my self interest) to pay them something. Unfortunately who are willing to become your impromptu Uber are hard to find.