I think you may be searching under Ford Mustang instead of under the Ford Shelby GT500 listing.
Try this link for the Shelby GT convertible.
TMV for my zip code for a Vista Blue one (nice choice!) is $55,857 loaded with everything, btw.
Wikipedia talks about a Shelby GT version of the Mustang; maybe that's what your dealer wound up with. These were modified by Shelby in Vegas and wouldn't show up in a regular Mustang trim listing.
Here's another discussion that may help a bit. I don't know how you'd price one if they are being modified by Shelby in Vegas, but try asking in there:
Article Comments - Ford Mustang Shelby GT vs. 2008 Subaru Impreza WRX STI
Please click Answer this Question to continue this thread - maybe a Mustang fan will come along soon.