As much as you can get for them.
Seriously, ducks [or other poultry] don't have an established value likw gold or oil. Their value is determined by agreement between buyer and seller.
If you sell them for $100 each that's what they are worth-if you sell them for $1 each that's what they are worth.
Dillon, What NY is trying to say is, if someone is wanting them really bad, then they will pay more for them and he would be glad to jerk their arm off! LOL In reality good mallard ducks in good health should be worth $40 - $50 a pair. BUT....if nobody wants them, then they are worth what you can get for them! Supply and Demand is everything.
I looked around on the internet and found some mallard biddies in South Carolina a guy was selling for $3.50 each and he had lots of hits on his offer. Adults would probably be considerably higher. Those are the closest ones I could find to North Carolina. At 3.50 for babies, my wild guess would be fifteen for adults. Try to check your local market bulletin if you have one. That will be your best indication.