I don't usually sell mine as ducklings but 3.50-5$ is the range i have myself paid and/or seen. Many times you'll get a deal like 2 for 7$ or so many at this price.
Most don't sell sexed so that doesn't play a role until their old enough to know through non vent sexing methods. As muscovy age colour and size, sex do play a role in cost. Adults varying from 10-15$ a drake to 15-25$ for a duck.. pairs can be 50-75$ depending on colours and quality.
This will all vary though, depends on the area/market.