Guys what's the lowest you can charge a client?

So I recently started having communications with a client. The clients' intentions were to rank for a low competition keyword. My fees are based on my hourly rate and my estimated time for the project. I have charged as high as $300/hr for big corporations. The least I charge is 15$/hr. Do you guys ever go lower than this? And if yes, how did it work? Note: Am not an agency.