I, personally, wouldn't pay more than the gun is worth, standalone price, for something like that. I own an AC 556 and it is just that, an AC, no more, no less.
+ 1 , only markings that I have seen give added value to nfa have been movie guns, like the ak's used in red dawn, something cool about the actual ak's with wolverines marked on it and notches LOL! Just get an ac and add a sage scar stock, best mod I have done to mine.