SO, how much is that guy's friend's car work that started this post??!?!?!?!?!!??!?!!?!??!?!??!
Hagerty lists the 1968 Mustang GT/CS with the C-code 289 at $33,500 in #3 (good) condition.
They list the standard 1968 Mustang coupe at $24,800 in the same condition
That indicates a 35% increase for the CS over a regular Mustang
Interesting, I am unable to find a value for any T-code 1968 Mustang, but obviously would be a deduction in value from the C-code cars.
This thread describes a car with essentially no optional equipment other than being a California Special.
So I guess whatever a basic 1968 Mustang 6-cylinder, 3-speed manual coupe is worth, plus 35%, if you want to put a number on it.