The LS6 is where the biggest cost is (and a convertible is very expensive). The LS5 is still very desireable and condition and originality will mean everything to a collector. For someone wanting to own one i'd say pay what you feel its' worth to you. I have seen them sell for that large a price spread too.
Anywhere from $35k-$50k would be fair if it's a very nice car (though maybe not showroom).
Sean
Thanks Sean....that's some good information.
My brother already bought a Mustang convertible from this gentleman and got a very good deal. The guy has several cars, is old school does not use a computer. He also does not want to deal with people coming and poking around at his stuff. He's want's to get rid of what he has left now. He says he'll take 25K cash for the car. We're going to go take a close look at it this weekend.