If any builder/seller has an exclusive right to sell agreement with a realtor, the commission is probably contractually pre-determined. If the builder has an "agency" agreement, they may have retained the ability to sell on their own without paying a commission or paying a reduced commission if the realtor did not bring the buyer. If you have not already made an offer on the property, you may want to consider hiring a buyer's agent to work on your behalf. A good buyer's rep (like myself) will help you determine whether the price on the property is fair and a strategy to get what you want at a price you're comfortable with. If you were going to court, you'd hire a lawyer; if you were sick, you'd go to the doctor. You're buying a house - one of the largest investments of your life. Hire a professional. Good luck.
Certified Buyer Representative
Senior Real Estate Specialist
Century 21 Princeton Properties