Ah yes, the eternal GT/CS vs HCS debate. Here's my take on it. For those of you who remember the flack I took for the Nissan opinion get ready for another onslaught of rebuttals.
In our minds the HCS is one of the rarest cars on the planet so it should be valuable but, because it is so rare, it is also unknown in the gerneral classic car market. It is also not well known in the classic Mustang market. Supply and demand would tell us that such a limited supply should demand high prices. The kicker is that since so few people know about the cars the demand is even less than the supply. That could be one reason for the lower prices.
Another reason might be that, in my purely personal opinion, the GT/CS looks better. The California Special script adds a lot to the looks of the car. And, again just my opinion, I think the "GT/CS" on the scoop looks much better than the HCS emblem.
I think that if we parked an HCS next to a GT/CS and asked folks which one they would rather own (based on looks only) the GT/CS would win over 75% of the time. Better looking cars are worth more.
A third possible reason for the lower prices is that some people may look at the HCS as a lesser version of the GT/CS since it doesn't have the script. It could be looked at like Shelby making a couple hundred cars and leaving the GT350 or GT500 emblems off. Ok, maybe not the best example but I hope you get my point.
One last reason could be that the Californiia Special production numbers of 4118 include the 251 HCSs so even in our own community we tend to marginalize the HCS. Why are the HCSs included in the production figures? Shouldn't we separate them?
One more example then I'm done. The 6-cylinder GT/CSs are quite rare so, using the rare=valuable logic, they should be more valuable than a 289 version or a 302 version. I guess it depends on how one looks at it but I would say that in most people's minds production numbers for these almost-identical cars don't mean squat.
Most car buyers make their decisions with their hearts inistead of studying production figures. That said....I still believe that the HCSs will eventually become quite a bit more valuable that the GT/CSs but it will take educating the automotive community one person at a time for that to happen.