Depends on the type of project and the type of service you are providing. Do not expect to make a lot from twitter since a large number of users  participate in that. On the other hand, signature campaigns are more rewarding but the downside is that you can only participate in one signature campaign at a time. As per my experience, the earnings per campaign vary from 20$ to 1000$. I have never ever received more than 1000$ from a single project since I started working on the forum, and those 1000$ too were from a long signature campaign. I hope this clears your doubt.