It would be beneficial for you to find out how many homes this particular builder has on the market, and how long the other homes have been out there. If they are selling other homes quickly, they might not be in a huge bind, and you won't have much leverage. But if all their inventory is stagnant, you will have some leverage.
What I would do is try to figure out how much it cost them to build the home. You can usually find this out through your county office, as they require permits to build. Next, I would attach a cost of capital (interest rate) used to finance the build. You will have to see what rates were at the time they began building and use that rate. I'm not sure if they pay property taxes where you live for homes that haven't sold. But it would be nice to check. Basically, you need to figure out all of their costs that they are paying per month currently while the house is just sitting on the market.
If this is a sizeable number, which it most likely will be, you can use that knowledge in your negotiation. You can say something like, "you're going to be at my price anyway in x months due to the costs you are paying every month, so why don't we just meet at my price now and save everyone the hassle", etc. But the key is finding out how much it is costing them every single month that that house sits vacant!
Good luck.