its all going to depend on 1) demand of your area - the more demand the higher the price. 2) the quality - better quality animals are going to be more expensive.
A hatchery duckling 3-5$
Breeder 3-10$
adult
hatchery- 10-15$
breeder - 15-40$ depending on breed, age, sex, and quality.
a good quality adult WH hen - 15$ is a very good price even here in my area where ducks aren't in high demand.
If your looking to sell - offer a bit higher than your expecting to get back, say 20$, then lower every few days/weeks 2-3$ till you get someone to hook and buy.,
If your looking to buy @ 15$ - offer 10$ if you can come pick it up immediately that day...(i've gotten many good deals doing this off craigslist) they may or may not take it - but you can atleast say your tried - and still go get it and pay the 15$.