A rabbit that dressed out to 4.5 pounds, was probably about 8 pounds live weight. So if the going rate is $2/lb live weight ($16) plus the processing ($4), then about $20 for that rabbit sounds right.
A live rabbit over 6 lbs, is considered a "roaster", not a "fryer" and they don't bring as high a price, although many of the European customers like the larger size. If this is your first sale, I'd start at $4.99/lb or even $5.25 and see what sort of demand you get.
Or you could just price them all at $20/each and not worry about the size difference. Some people like a flat rate better.