Depends on your area..
CK's lose value as they get bigger, because LFS have a hard time reselling them due to their potential 3' size. Most people who want a 3' knife, already own one, and probably aren't looking for more..
A common CK that big at my LFS would have a sticker price of $99.99.. so does the Royal CK for $99.99..
They have been there for 6 months in a 55g tank, if that gives you any idea..
I would say, $50 max.. Like I said, people who want common big fish like that, already have them..