You can ask a Realtor to do a CMA, which will be quite accurate.
If you are considering a Shortsale, you will need a Realtor in your corner anyway.
There are two ways to get a good estimate and even that is fluid depending on recent completed sales of comparable properties plus or minus adjustments. One is to get an appraisal which will cost you. Another one is to speak to a Realtor experienced in short sales.
I am not sure why you need an "accurate value estimate". The fact that your home is underwater in value and is in the foreclosure process already tells me two things: 1. You don't have the means to make the mortgage payments and most likely don't have money to bring to the closing table either. 2. If you do a short sale the value of the house is not your biggest concern anymore.
Damage to your credit has already been done. Now the question is how to best get out from under. Do not let the property go to foreclosure in Florida. It's not the end but may just be the beginning of a 20 year period where the lender may be able to come after the deficiency. Also, if your name is on the loan docs you are liable even if you quit claim the property to someone else. This is not legal advice!I am passing on professional experience and would urge you to get legal and or tax advice before making a decision.
Too many variables in my opinion and there is still one more. Right now a short sale is just another kind of real estate transaction; there is no stigma attached to it. Also, until the end of 2013 the deficiency forgiveness is not considered income. Do you know how it will be in a few years when due to some life circumstance you may have to move? We don't know how long it will take for values to come back where they were before the drop! Doing a short sale you may qualify for a new mortgage in approx. 2 years if you have the income. Lots to consider .....