We asked a member of the Edmunds Data Team, and below is his reply. Hope this helps.
"The Firebird SLP Firehawk was a factory authorized third party conversion, as a result the content and cost of the conversion was never published in a Pontiac dealer order guide. Our used vehicle data is based on information provided by the manufacturer, this limits us to basing our True Market Values on data that can be verified through a dealer order guide. Third party conversions can vary widely in quality of execution, in addition some conversions can vary in cost depending on geographical region. What it comes down to is that there are a number of variables connected with third party conversions that make it extremely difficult to arrive at a meaningful resale value, so we limit our evaluations to factory-installed options, where prices and quality are generally standardized.
Our policy on third party conversions should not be taken as negative commentary towards any particular builders, the quality of work done by some third parties can actually exceed that of the vehicle manufacturer, but as stated earlier there are just too many variables to consider. The very nature of most third party conversions aims them at a niche audience, so personal preference becomes part of the many other factors to consider.
With the SLP Firehawks being specialized vehicles, they would be the most valuable to those who are somewhat knowlegeable about them; in order to arrive at a value we would suggest checking the classified listings of several vehicle-specific forums or any related eBay auction results. By clicking on the "Advanced Search" option from the eBay Motors home page you can search for completed auctions:http://motors.search.ebay.com/ws/search
Included below are some links to a couple vehicle specific forums:http://messageboard.firehawk.org/viewfo