General Contractors Fees for Building a New House

[Audrey C.]

How much are the typical contractors fees to build a moderately priced home from the ground up and around $300K selling price?  What is the % and if so, based on what?  How do you give incentives like profit sharing, after expenses, to motivate the contractor to cut costs and save time?  If so, in what percentages? example 80/20 or 70/30?  Please help.  I have the lot and the financing ($$) but need suggestions on how to pay the GC.