From the information you have given to me I would feel at ease with a 540 to 550 offer The builder should not be offended at this price he will counter with an offer and you can come to a reasonable and equity price you both can live with.
Lets have some parameters to look at then I can better answer the question
If you only have older comps to work with you can come up with a figure for depreciation using the age/life method. Typically a home has a lifespan of 60 years and depending on how new a home is or how it is upgraded can effect the effective age of a home. E.G. A 10 year old house with very little maintenence and shows as a worn house could actually effectively be 15 years old. While another 10 year old house that has been very well maintained showing very little wear could effectively be 3 years old. So based on this example the worn house has depreciated 25% while the well cared for house has depreciated 5%. So, if in your area a builder can build a custom house for $250 per sqft and for example the house is 2,000 sqft the construction cost would be $500,000. Now take into consideration the 2 depreciation scenarios I have given: 25% x $500,000= $125,000 in depreciation and 5%x$500,000= $25,000 in depreciation. So the well cared for home would effectively be worth $100K more than the house that was in rough shape.
I hope this helps you out.
Best regards, Dave
I always try and talk to the listing agent before writing an offer so that I can get a sense of urgency, seller motivation, and try and dig up as much info as possible before placing an offer. This is why working with a good agent is crucial as he/she will protect your interests, review the market, and negotiate the best price possible. If you have any additional questions or would like to discuss my answer further feel free to contact me.
Best regards, Dave
David Westall, Realtor
Tahoe Riverfront Realty, Inc.