I did some estimating when we were writing "services" contracts at IBM. There are basically 3 areas you have to address. "Materials", "labor, including travel" and the hardest to estimate "risk". That is the amount you have to add for things you forgot or what just pops up after you start. Underestimating risk is the one that kills your profit when you hit that water pipe or your helper spills the cherry kool-ade on the marble stair case.
