Wiki Consulting Opportunity

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I have recently been approached with an opportunity to do some ongoing consulting with a revenue cycle management company. I am trying to determine the fees that I will be charging and I need a little help...

Specific projects (writing a compliance manual, creating software user manuals, etc) will have a set price per project. I will also be available at all times to answer questions from the support staff in regards to coding and/or billing matters. We have decided that I will be paid a retainer fee to be available for these questions.

My questions:
1. How do I determine what the retainer fee should be?
2. Does anyone have any other ideas how to set the fees for this office?

As always, thanks for all of your input and help!!
I think you need to determine how much you think you are worth. I change $45 per hour and get consluting work, the teacher I had in school charges $65 per hour and is always super busy with consulting