Renee's answer is 100% correct.
I worked for a new construction builder for 5.5 years and know for a fact that new construction prices are always inflated dramatically. Depending on what the company's margin is, pricing typically starts a lot higher than what it actually sells for; especially with smaller, not so well known builders in Northwest Philadelphia.
Having a buyer's agent represent you will benefit you in a number of ways:
1. It won't cost you a dime. The builder is the one who pays the buyer agent's commission.
2. A buyer's agent will work diligently on your behalf not only to arrive at a fair sale price, but will make sure the builder follows through with all promises regarding construction, warranty, etc.
3. They take a lot of the work and stress off your plate when working with a builder. It's a long process and can be frustrating at times.
If you still need some help understanding why this is so, please let me know and I'll help you out. Best of luck to you!