How much a builder will discount also depends on whether it is a small local builder or a national company. Sometimes publically-traded companies like Pulte or Hovnanian are trying to meet sales targets at the end of a quarter or end of a year and are willing to give bigger discounts then. Smaller builders can't do that.
Have a Realtor pull settled comperables for similar homes. Make your offer based upon what others of similar size and location have sold for.
You may want to start at 10% below that figure.
The best thing to do is have your buyers agent pull comparable properties that have sold and see what they are closing for. It may depend on the situation as to whether the builder can come down on the price. For example; what if the seller has it priced at the lowest price they can afford without having to bring money to closing and they don't have any additional money to bring to closing. They would have to get permission from the bank to short sale the property which would damage their credit. You may also want to consider having a home inspection on a home that has been sitting vacant for over 2 years. Homes are not made to sit empty. If you have a buyers agent they can assist you with the answers to what is a best offer price. If you are not currently working with a buyers agent I would be happy to look specifically at the comps for the property you are looking at to help determine the best offer price.