I would say they are not worth. I participate in many twitter social media bountys. Some of them from the beginning. But the stakes you receive there are always like 20$¨or less. So for just retweeting maybe not such a lot of work. But some of them also require to do own post about the project.
Can you specify for what time frame you received this 20 dollars, did you do the campaign a month, a week, or a day? If it was per day it is not that bad at all.