You're forgetting (or you just don't know, actually) about the costs for tooling and setting up the machines to stamp these out, plus sourcing the color dyes, and of course the R&D involved to come up with the colors and the band design and also marketing and logistics costs. It's an 8 figure $ operation at the very least.
All in all the margin is probably around 20-25%, typical for retail products
I think you’re right in that everyone forgets the upfront R&D, shipping, packaging, marketing, cost of returns etc., but I’d say the margins are much higher than that based in the price of the knock off bands all over Amazon and eBay etc. Except for the R&D and presumably lower material and QC costs, the knock off bands sell in multipacks of 3-5 bands or more for under $20.
These are highly profitable items for Apple, especially when sold direct to consumer vs. a separate retail outlet. I’m not complaining about this (Tim Cook has to eat after all and no-one has to buy a watch band) I’m just saying that the margins are likely much higher than 20-25% typical retail price.