Projects rates can be good but clients often get the wrong idea. They feel that they paid a good sum of money and deserve whatever they want, even if it's not what they specified. Because your time is not directly linked to their dollar they will not think about it as compensation for time. I try to avoid writing quotes for this reason. I give an estimate for what the project should land around if everything goes as specified. But clients nearly always change specifications on you. Charging per hour takes away any claim they have to free hours.
I suggest you read
Lowengards guide to writing quotes. While a little tongue-in-cheek it shows the attitude to quotes you have to have if you want to avoid being screwed.