Previous answers have great info. In addition, for single family detached homes, refrain at all costs from buying into a community where there is only 1 builder. That builder has control over all the information and they have no competition within the community to keep their prices in check. Some argue that communities outside of the 1 builder community can keep the builder in check, but not nearly as good as a community where you have 3 or 4 builders competing for your business.
Lastly, be sure you absolutely have to have a new home. In the scheme of things, all other things being equal, buying a new home is a poor investment relative to buying a home that is just a few years old. Yes, in a new home, you get that new home feeling, and if you're really lucky and have money, you might get some customization, but the newness will quickly wear off, not only due to "wear", but due to the fact that you'll quickly realize that you just paid considerably more than what a similar (nearly identical) used home would cost that's only a couple of years older.
New homes have to be built at current costs for land, labor and materials.
You so homes to compare them to - you can compare them to others in the neighborhood that just sold as well as similar resales that are 1 to 2 years old. There are always "comps" for new construction in an area such as Herndon or Ashburn.
And, as with dealing with individual sellers, it depends on the builder's degree of motivation.
But you certainly should try to negotiate. It never hurts to ask.
One other tip: Make sure you visit new construction with a Realtor. The builder's built in an expected commission if there's a Realtor involved. If not, the builder just pockets the money. And a Realtor can help you in negotiations. It's well worth it.
Hope that helps.