New construction homes are always priced higher than resale homes. Think of it as purchasing a new car vs. used car. Of course, one can argue that sometimes resale homes offer more than new construction homes, unlike used cars :-)
A good appraiser should compare new construction homes to new construction homes and resale homes to resale homes.
I hope that you have a good buyer agent because your agent should be your best friend right now in helping you decide the VALUE of the new construction home vs. comparable resale homes in the area.
In the end, it is between you and the seller/builder to agree on the price - unless you are locked into the price by the contract that you have signed with the builder, in which case you have made the decision when you did sign the contract.
Do you have an appraisal contingency? If so, then perhaps you can get the price lowered - that would be good! Your agent can guide you through this.
If that doesn't work your agent can challenge the appraisal if the two of you feel it is unfair. Any good agent will know the appropriate ways to do this. If you're working with a new home builder - in particular a production builder - I am surprised they aren't handling this for you.
If the appraisal isn't getting changed, and you don't have an appraisal contingency, then the issue goes to how much you have to pay and will you still be able to close. Sometimes if you have the money to pay the difference, it will lower your reserves so much that you might not be able to get financing. If that is the case, and you have a financing contingency, you may be able to get out of the contract without penalty. That's kind of stinky, but it is better than the alternative of having no contingencies and not being able to close, risking your deposit and possibly more if the builder was to sue. Yucky, yucky, yucky.
Are you not working with a buyer agent? This should be spelt out in your contract. First, the seller has the option to reduce the sales price to the appraised value. Then you have the option to come up with the difference in cash or void the contract. This is assuming you have the appraisal contingency in force.