Which species are you talking about? The Trachelyopterus galeatus? If so, these get to 8-9 inches. As for price, I cannot comment on that as I live on the other side of the world.
can honestly say i've never seen one of those before. I've had and seen Auchenipterichthyscoracoideus and they generally run from 8 to 12 here in CO. so i imagine around the same price for a common woodcat.