I Read a post today by Steve Zelle on his website, concerning why design is not something that is quantifiable to the extent that you can say designing Project X will take Y hours and therefore cost you Z.
He obviously makes his point far more eloquently than I could - give it a read.

Adrian Bornea
August 15th, 2010
I had a client like that recently. Who asked me to design their logo billing me per hour. That's insane. Creativity can't be billed per hour.