It’s a tough one, but it nets you something in the ballpark of $200-250 for first-time completion, similar to the heist payouts in GTA Online.
One of the story missions currently available in Red Dead Online is titled “Kill Them, Each And Every One,” and it involves you and your squad storming Fort Mercer with helmets on your noggins. This is by far and away the easiest method of making quick money, though it comes with a catch.