Ok,
I'll go first....
A Cobra CSX2000 or CSX3000 is always historically significant. Racing adds to it however there is good racing and bad....
289 or 427 values vary as does their history. Say CSX2000 and CSX2345 are priceless and 2049 is a highly questionable car. A car like CSX3002 or CSX3009 are also a significant 427 car's so they would value higher than most street 289's.
Short answer is the specific car, the race history and drivers of that car, its originality and the condition is all in consideration for its valuation.
There is a fare amount of information as to selling prices available. Also a call to SAAC might aid in your favor.
If your client has a wire wheel 289 street car and says its a 289 and thus more valauble than say CSX3002, they are just overly excited.....
Each car has its own history.... 289 and 427....
You can always call as my # is below...
Let the games begin!