Prices of duck eggs vary by region, purpose and availability. Depending on where you live, what you want to use them for and how easy it is to find them around you.
Hatching eggs are obviously more expensive than eating eggs and usually get shipped. They can range from $0.50 to over $50 for one egg. Plus the cost of shipping.
Eating eggs are usually sold for about $3/dozen up. Maybe some people from your region should answer that question for you then.
That's what set my final price. Three people showed up at the same time and I only had one dozen eggs. Instead of taking a few each(for free) they started bidding on them. $10, I was stunned so I cut a buck and a half off that bid and set my price.