I just know how I feel being on the other end. I have a friend who asks tons of favors. At the beginning I made it quite clear I actually do expect something back. If I do her favors, she will do me favors. She said she was OK with that. Now I know this situation differs from the OP, in this case she had me do maybe 100 favors for her. She always said she will do something back. I finally ask for 2 favors, she says no to both. Then she said fine she will pay me 10 dollars for all my work.

I wasn't bothered with the ridiculously small amount at first, but that she wanted to buy her way out of the deal. Mostly this deal is not outspoken like ours was, but it is there nevertheless. Money doesn't make up for effort and time, because a favor is not work.

She knows I get "annoyed" now because I refuse to help. But she still thinks I am unfair and asks some more. I don't need exact fairness but 100 to 0 is just too off according to my rules.

The people taking money for favors are those in need of money, then they can set social norms aside. But if they already have money, offering money will actually not help them in any way.

And in my friend's case, I think she is totally unaware if she had hired someone for what I did, she would have paid about 2,500 dollars. But for some reason, people usually pay friends less than they would strangers....