I have a deal with some friends in which I'll take care of their two ducks (White Layers) and give them 3 dozen eggs per week in exchange for them to pay for all of my feed costs (6 ducks total). In the end it equals about $5 per dozen eggs. Depending on where you are, ducks eggs can go as high as $12 a dozen. I called a local farmer in MD that sells Pekin duck eggs and he sells his for $7 a dozen. You can also look to sell to restaurants which will pay top dollar if your ducks are organic, non-GMO, free range, etc.
I would also assume, your cost should be associated with egg size/duck breed. I have heavy weight ducks so their eggs are very large (85-+100g).