With the great summary that "Administrator" has brought up on to answer your question, why would you still consider offering lower than what the listed price is on a short sale? Of course you are free to offer anything that you consider as the value for that property , in your opinion, however in real market, It takes forever for lenders to get back to an offer on a short sale (I am waiting for one for 3 three months, and had another one since Nov 2008!). Unless it is a confirmed short sale, going near short sale properties specially at a price lower than the asking, in my opinion, is a waste of time is more of a waste of time.
Another point to consider is that most of the time in order to make the property attractive and looked at the short sale properties are listed lower than the average market analysis.
Just like Richardo said, talk to your local agent for a reliable CMA but consider looking at the REO's (Bank Owned) for more reasonable negotiation options.
Century 21 Hallmark Realty