Personally, I always set my hourly rate slightly above what I make from my primary means of employment. After all, this work is done in my spare time that could have been otherwise spent with family, doing recreational tasks, cleaning the house, etc. On top of that, freelance work is sporadic and unreliable, thus necessitating a higher operating cost to turn a reasonable profit.
Now, if someone is going to give me a bulk of work at a time, I usually lower the rate to the same amount that I make through my primary means of employment. Similar to the concept of purchasing in bulk to receive a greater per item discount.
If you're salaried, divide how much your annual salary is by 2000. The result is the approximate hourly wage you're making. I generally set my contract rate between 15% and 25% higher than my base wage but the actual choice is up to you about how much your time is worth to you.
Hope that helps!
*Edit - clarified the type of salary to use in the formula.
