Keep in mind there are two approvals you need to have to be able to create a sales agreement. The seller and their bank both must approve of the offer. The seller can only qualify for "short sale" status when a financial hardship exists. With this said, they probably cannot afford to go much lower than their listed price. Plus, the bank will need to approve the final sale price since it will be less than the amount owed.
Our recommendation is to keep it real and base your offer on real facts that reflect the current market trend in this location.
Many people think a short sale or foreclosure is a 'DEAL' when in fact that is not always the case. Why just look at short sales or foreclosures (which take MONTHS to close and are as is sales) when you can look at everything on the market?
Get a good buyers agent and work with them. The world is your oyster, it is a buyers market right now and interest rates are low. If you are a first time homeowner, you also have tax incentives.
First I would recommend that you work with a Realtor that can give you specific information as it regards to this particular property and your local market.
But, essentially what the bank is going to do (with your offer or any other offers) is to get a 3rd party "price opinion or appraisal" to make sure that whatever offers they are considering, the property is being sold at a comparable market price.
In effect, they are going to want to make sure that the bank is losing as little as possible.
I have seen short sales sell at $10,000 below asking price, and I have seen them sell at or above asking price.
There is no "standard percentage".
Hope this helps. Good Luck