In my experience, builders are easiser to lowball than REO banks. It all comes down to how many buyers are looking at those houses and how long the builder has been carrying them. I think builders are the group most likely to respond favorably to low ball offers. They are not emotionally attached to the home, they need to turn inventory. Their financial performance is measured in part by inventory turnover.
One thing to realize about the builder is that your low ball offer will impact his other homes in the area. So, get creative. Are there things you can ask the builder to pay for, such as closing costs or Mello Roos tax, that won’t lower the comps for his other homes too much? Those things can effectively lower the price you are paying while preserving his comps at the same time, not to mention your own after you buy. To that end, a builder will rather give you more upgrades than lower the price if he has lots of homes still in the area to sell. You can use that to your advantage as well.