I know, I know. Every Res. appraiser hates the cost approach. But check this.....Get the builder to level on exactly what he spent, building, lanscaping, septic and well (if present), labor and all..Then the lot value and any building permits, etc. etc.
Assuming this is a garden variety home (i.e.-no exotic architecture plans except something out of a catalog, you know, $150 from Bitter Homes and Garbage Magazine. Properly, I think in my area, 7 - 10% allowance for contractor profit, then maybe 10% for Risk and an allowance for time. I.e.- He has (likely) borrowed money at commercial rates for 6 mo - 12 mo to construct the house and allow time for a sale. Say, 6 mo. to build, 6 mo. to sell [that would work in my area except in Lakeside because land is scarce and houses bring a premium]
That number should be a starting point. Find a similar new subdivision if possible and then check the prices. I prefer to adjust lot prices first (i.e.- land values may be different depending upon lot size etc.) Subtracting out the land value, the solds in another subdivision should be close in $/SF.
Clearly, the house cannot sell for less than the guy has in it, or it is a disaster. It is a hot market indeed for it to sell for more than 20% above cost + lot in my opinion.
Using Marshal & Swift in lieu of actual costs remember they include a contractors profit and allowance for time.
Your comparable sales needs to be truly comparable for the improvements in age (new), style, and size as well as quality. The lot values can be adjusted out.