This is a tough question to answer. The amount of money you can make with micro job sites depends on several factors:
1. Your free time. The amount of time you have to work is the main factor that affects your earnings. People with less than 4 hours per day free to complete orders will not earn as much as people who work 8 hours per day for such websites.
2. Your work quality. On the internet work quality is the most important factor. If your work does not have supreme quality, the client may accept it once but will probably never order again.
3. Your work niche. Some niches have more demand than others. Everyone needs articles written but not everyone wants to have Myspace templates, especially now that Myspace is declining hard.
4. Your effort on getting new clients. Facebook is the perfect way to get more clients and orders. Post your profile link to Facebook groups related to the services you offer and then just wait for new orders. This action is not mandatory but people who are not getting enough orders should do it to boost their revenues.
As you can see, making money with micro jobs is good to have fun but not as good for real professional work as the site webmaster charges too much for the service. Frankly, 5% of your work is too much but 20% is really outrageous. It seems that by working using these sites you are working for someone else and not for you.