Find an agent to provide a CMA for you using recent data--that will help to determine the market value for this home, and make your offer accordingly. Understanding what is going on in the market currently will be your best guide in making an appropriate offer on this home.
Best Of Luck,
You have received many good answers to this question and all are correct. Two things that you mention strike a unique situation for easily determining what a "good" offer is. First is the fact that you mention it is a "new" home. If it is a new home and not just new for you. There may not be many good comps to determine comparible value. In todays market there are few new homes being built and fewer being sold. Possibly one of the reasons for the time on market. Bulders and banks are not building many spec homes today. The price of new homes can vary significantly even in any neighborhood based upon the construction, features and location. New homes may also involve seller/builder concessions. The status of the home complete/ partially complete, appliances included, even things that the builder may be willing to do to get you to buy, ie paint, change some of the features, make additional improvements.
The best answer is what are you willing to pay for the home assuming you have done good research on other homes and what they are on the market for and or have sold for recently even though they may not be exactly new.
Contact a good realtor and get some advise based upon the specifics of the house in question.
If I can help pleasae give me a call.
I have seen sellers not budge on a dime, others take offers tens of thousands of dollars less.
The motivation level of the seller and their ability to make financial commitments seem to be the reason.
If this home has been for sale for 15 months the sellers are not listening to the market. It is either the price, location or condition holding up this sale.
If you offer over that, it may not appraise.
Definitely disuss this with your agent...they will be the best source for guidance.
Realtor, GRI, CSSN
John Hall & Associates