Mine sell exceedingly well at $5 each. And yes, the price goes up as they get older. By five to six weeks, I ask $8-10, depending on the bird.
So far, the only time I've had ducklings more than a week was when I hadn't decided whether I wanted to keep them. They sell out before they're hatched.
I allow folks to pick up at 24 hours, but I can totally see waiting a bit longer. I ended up giving one duckling away with another purchase because she seemed weak to me, and I didn't want them to be a duck short if she didn't make it. In another 24 hours, I probably could have identified whether she was truly weak or just a slow starter.
As with many things, the best way to figure out your price point is to pick a number and get started. If they sell like hotcakes at that price, try raising it a bit and see what happens. When you hit a price point where you're having trouble moving them out, lower it a bit again and you've hit the sweet spot. For me, I like to have them all sold within a week, because I don't like having multiple batches of various aged ducklings to care for--it's a hassle. So I keep my price at a level where they sell quickly. If I were more concerned about $ and less about hassle, I'd probably raise it a dollar or two and they'd sell eventually, just not as quickly.
Good luck--