Many buyers are in the same boat and there was an article within the past two weeks in the News or Freep about builders putting in less expensive amenities. That may be an option.
The reason I chose to answer is that I currently have 2 clients that were in your situationwithin the last 2 years and decided to continue with the purchase of the new home because they would lose their deposit. Both clients would love to go back in time and forfeit their deposit and get out of the new home purchase. Hindsight is always 20-20.
Some of the other answers mentioned the appraised value and I agree that the appraisal will be important. Another question is the inspection - do you have the right to exit the contract if there is a problem with the home? Again you need competent guidance.
Each situation is different. Your builder has a lot riding on your transaction and might suffer great finanical harm from your default. But you have a lot riding on this too - more than you may realize. Consider all your options. Don't do anything without an attorney to guide you.