I would only add that if a builder has homes in contract of the same model, the probability of builder concessions is slim. When builders know a particular floorplan can command a certain price, they are reluctant to negotiate. Good luck.
There's not set rule and the percentage can change based on the model of home, shifts in the economy, demand, etc.. Steve D below gave a good answer to a similar question in this post a few hours ago: http://www.trulia.com/voices/Home_Buying/Tessera_for_bedroom
Here are a few more considerations to be aware of when buying from a Builder: http://docs.Steven-Anthony.com/BuilderRealtorAgent.pdf
Everything is negotiable. How much negotiating depends upon how far along the project is in sales - when starting out they don't want to drop prices to set a bad precedent. However, upgrades, credits for closing costs, pre-paid HOA fees (if applicable), parking, etc... can often be obtained because it doesn't hurt the public perception on sale price.
Get an agent/broker who is familiar with the project or has experience with representing buyers on new construction projects to help you as the process is different and so are the purchase contracts.
Lance King/Owner-Managing Broker
You most definitely can negotiate terms, price, upgrades, etc. If you use a Realtor to help you, you will probably save more. The Realtor is going to know how much you should probably be paying for the home and be completely on your side, unlike the builder who is all about making as much money as possible. Remember to have the Realtor take you or meet him at the builder trailer or model and sign you in the first time you go. If you show up without the Realtor, you will not be able to utilize the Realtor's services to help you negotiate. And don't believe any of the builders who say that you can save if you don't use a Realtor. The builder has already set aside funds to pay the Realtor built into it's costs.
As far as how much, that is all negotiable. It depends on the builder, how long the site has been up, what incentives they are offering. If you need help or more information, I would be glad to help you out. Also, if you need info on schools and demographics for the area such as crime and other stats, I would be glad to send that to you so you can get a good sense of the area. Good Luck.