Nothing at all unusual about a 788 chambering in .223; it's the "NIB" designation, thats the oddity.
Used to be,and not to long ago, a good to excellent 788 was worth $275 to $325 in my opinion. Now, they seem to be bringing a slight premium, so I would be inclined to believe a good to excellent 788 to be valued from $350-$425, that "NIB", if true, might indeed bring $450 or better, not out of my wallet, but someone who really wanted one might pay that?