The answer to your question is about 50% of blue book/Retail. I have owned many Salvage/Rebuilt title cars and I would not pay more then that for one.
I would also NEVER,NEVER,NEVER buy a car that has Flood/Water damage you will always have problems with them and the Boxster's ECU is under the drivers seat.water damage on a Boxster is a death sentence. If I were you I would run,not walk away from this one.
Good luck
