New constructions are hard to gauge right now. The reason I say this is because in my area, builders are taking a real beating. As a Realtor who deals mostly with existing home sales I advise my clients not to regard any offers of less than 90% of the sales price. New constructions are different because they are held as stock that needs to be moved off the shelf. On top of that, as I said already, homebuilders have been having a hard time of it for much longer than the rest of the market. I would suggest that you get yourself a Buyer's agent and as them to search the sales price of new builds for the last three months. The market is shifting and changing so much that I don't think sales from six months back will present a legitimate picture. I would (from a conservative point of view) feel that an offer of 80% of the list price would be a good place to start. There have been some articles that suggest starting at 50% but I don't recommend it. If It were my seller, I would advise them not to even acknowledge the offer and if you really want the house and this happens to you, it would mean having to make another offer with cap-in-hand.
I strongly suggest that you seek out an accredited buyer's agent, (one who holds an ABR designation) and consult with them about what the market is doing and make your plans from there. You can get a new build for a very good price, but sellers are not going to give anything away for free.