You should be working with an Agent. An Agent can pull all the homes that have sold in that subdivision to give you a comparison or CMA (Comparative Market Analysis). This will give you the information you need to make an educated decision. Just going to the builder (if it is new) will not give you a 'good deal'. They work only for the builder. Also an owner/seller may not give you all the information you need. Call a professional.
Feel free to contact me! I live in Herndon and would be happy to assist!