I read the title again...and again. I wouldnt take it for free. I wouldnt take it if you offered me $50 to take it. I have no use for an 18" fish, no matter the type. I like how Oscars look and how they are owner responsive, but they are poop factories and Id imagine an 18" is even more so.
If someone asked how much would I pay for a 30" dovii, Id say the exact same thing..and would possibly up my "I wouldnt take it if you offered me $50 to take it" quote to $100. I know what a fish like that needs and I couldnt offer it ideal conditions, even with a 265g tank.
Its worth whatever someone who wants it will pay for it, THATS IT. Plain and simple.
If I could find one, Id pay up to $5000 for a 1969 left handed Fender Stratocaster with a red to yellow sunburst. Would you? I doubt it because it doesnt mean squat to you. Just like an 18" WC (feral?) Oscar or a 30" Dovii is to me.
All this talk (mine and yours included) is subjective and, as you put it, "a useless contribution."