Since all the Stainless Steel watches have the same plain Stainless Steel base, the extra cost should just be in the coating applied (Space Black, Graphite or Gold). Since Space Black and Graphite are both DLC, their coating cost could be very similar.
I expect Apple introduced a Graphite Milanese Loop to match the Graphite watch. Interesting that they did not release a Graphite Link Bracelet and in fact kept the Space Black Link Bracelet while eliminating the Space Black Milanese Loop.
Space Black was I believe more expensive when the Series 0 launched. In fact, I think Series 0 Space Black was only available with the Space Black Link Bracelet at launch, so it was like $1499. I liked SB, but that combined cost forced me to get the plain Stainless Series 0 with the Milanese Loop.
And Apple very well could have just charged more for Space Black because they could - offering it only with the Link Bracelet somewhat supports that. And it would make sense why Apple charged more for the Space Black Milanese Loop at first and continues to charge $100 more for the Space Black Link Bracelet over the Silver Link Bracelet. I don't believe the DLC coating actually costs them $100 more. It's probably $10 more and they could eat it, but this is Apple and they don't eat margin unless they absolutely have to (like with the Milanese Loops, I imagine).
With Hermes, you're paying for the bands and the custom faces more than anything. I expect they kept SB primarily because they wanted to continue to offer variety - Hermes was plain Stainless only until the Series 5, let us remember.