He's worth whatever Carlton is willing to let him go for
What are you going to do if no one offers a first rounder, for arguments sake?
Considering that
1. He's contracted and so has to agree to the trade, can't just be shipped to the highest bidder.
2. Has publicly stated he will never wear the carlton jumper again.
What are you going to do, force him to play in your VFL team? Give me a spell, what a way to kill morale and dissuade any players that might ever want to come to carlton in the future. It would never happen.
By stating he'll never play for you again the club has been put over a barrel in this trade scenario. You'll simply have to agree to whatever club he wants to go to is willing to offer. He might give you a choice of 3 or so clubs that he's willing to go to, or he might say get me to club x and that's it.