If you had a full time job doing your consulting work for a company what would the hourly rate be? Gross that up by both sides of taxes, insurance, etc.
What is the going rate for your type of consultant work in your area? The rate for my area (rural-ish) will be different than a more urban area 100 miles away.
Are the other consultants that do what you do? Ask them :)
Do a google search for your line of work - they are a lot of websites that track salary trends.
When you do figure your hourly rate don't forget to figure in taxes - you will be paying both the employer and employee side, and covering your own benefits. The plus side of consultancy is the tax deductions that you can take - mileage, supplies, a % of home expenses if you work out of your home.
I do grant writing and non-profit management consulting on the side - I charge a basic hourly rate based on my prior salaries but grossed up for things I mentioned above. In some cases, depending on the finances of the organizations that I work for I will lower my hourly rate...I know what my flexibility is - they think they are getting a discount :) I am a little lower, rate wise, than some other consultants in my area but I am still trying to build my side work - so the lower rates are worth the exposure for me.
Here is an interesting article - http://www.entrepreneur.com/article/41384
Good Luck !!!!