Contracting/Consulting fees

I have an opportunity to do some contract work for another facility, which brought up the question of how I should determine what I should charge for my services, and how do others determine what they should charge.

Now, I want this facility to use me again, so I want to make sure I value price my services, but on the other hand, what they want me to do is fairly complex, and I don’t want to short-change myself.

So, how do you guys determine what you’ll charge a client? I currently perform this function at my current employer, but in this case I’d work from home over my existing broadband connection and would supply my own equipment.