It is important that you look at your contract and not give a huge deposit. Most builder contracts say that if you do not perform then you lose your depsit but if you are willing to risk your deposit and walk some builders will conede on prices than risk a fallout. New home sales contracts are written in favor of builders and the sales agents do not represent you, they work for the builder.
The problem is that if they lower the price for you what are they going to do for all the other contracts in their escrow pipeline? It then becomes a business decision for the builder that may not work out favorably to the buyer.
Sometimes your contract will be a price for a Lot reservation and then you negotiate on price later. One of the problems with appraisals on new construction comes into play with the value of upgrades and lot premiums. Since builders often times do not put these into the MLS an appraiser will not know if one of the comparable homes was a result of a builder concession or buyer paid improvement.
There is no harm in asking, you will need to understand what rights and duties you have in the contract should you decide not to go forward with the purchase. In addition to your deposit you may also have to pay for any upgrades or changes orders you agreed to.
All the best in negotiation. Also I donot see any harm in asking through agent or yourself directly.
Walking away while losing your deposit may be a pretty good option, especially if you can pick up a similar resell home for much less. Good luck..