It will often vary by location. Some specialty classes can draw a little more money. Typically the government such as the military will set a pay grade. Usually a university, city, or community center will pay around the same, often $15-30 per hour. Others you must treat as though they are clients and name your own price. If I need to pay an independent contractor, I look at what others pay in the area and set a fair, non-negotiable price, sometimes based on their qualifications as well.
The best rate would be one that provides a ‘win-win’ for you and the client. One that you feel comfortable with and not feel taken advantage of, AND that the client feels they are getting fair value for. Consider all of your particular factors that go into establishing your rate, consider your client’s budget (assuming you’ve had that conversation with them) and then set your rate.
When looking to work as a contractor for the government, you’ll notice the pay rate or range is pre-determined and is listed within each job posting. Each department has different rates depending on the type of job and the responsibilities of the position you are applying for, and it’s very difficult to negotiate your rate. As far as doing business with a local church or other not-for profit, room for negotiation is about the same. Because they likely have limited resources, they may expect you to accept a lower rate and view it as an opportunity to gain influence or exposure in place of a higher rate. I have found that working with a private company provides the best opportunity to set your own rate or negotiate the rate you wish to receive for your services.
I hope this helps.