John get a Realtor familiar with the area to help with the purchase. Builders normally do not want to break price but an agent may be able to get "upgrades" included.
I just had a new constuction that my clients signed a contract for on saturday and the builder included a walkout for the basement and Maple cabinets instead of oak at no additional charge.
An estimate of price per square foot will not reflect these additions but it would have cost my clients about 7000.00 if they just walked in and talked to the person that works for the builder.

I can look up the sold properties in the last six months to a year for you that are within a 1 mile to 3 mile radius of the general area or just use the zipcode itself if we dont have an address and then I can tell you the average sales price per square foot which would include the land also and then you just tell me what the square footage you are wanting to build or what quare footage it is already and we multiply that by the average square foot price and I can tell you what you should bid on the property to be built or that is already built.Then hopefully you will use me to represent you in the transaction and if not then I will do it for you anyways.

If you do not have an agent representing you, ask the builder's sales person to help. Then ignore the advice and go out and get an agent to represent you.

Do your homework. Ask a Realtor that is familiar with this builder and maybe not the one who represents them to help you. It's not as hard as you would think...unless it is a very unique property.