I would guess that the ceiling is partially set by other DC Comics from that era with lower print runs but high demand. For example, Batman Adventures #12 has a lower print than Man of Steel #18, and Harley Quinn is more popular than Doomsday. As a result, I would not expect Man of Steel #18 to even (ever?) come close to the value of Batman Adventures #12. Man of Steel #17 is an interesting case because there is much, much less Doomsday in Man of Steel #17 than there is Wolverine in Hulk #180, but for now, Man of Steel #17 has a higher price. What is that higher price for MoS #17 based on? It's not based on the amount of Doomsday in the book.
It's likely that Man of Steel #17 has a premium (for now) over Man of Steel #18 due to the fewer number of CGC graded copies for #17. If both books had similar CGC census numbers, we would likely see Man of Steel #18 with a higher price because it is the first "full Doomsday". That's an interesting situation though, because Doomsday "appears" throughout Man of Steel #18, but he's always covered by his wrappings. We don't get to "see" Doomsday until Superman #74.
I think it's the cover of MOS18 that collectors like so much. Great cover.
And yeah.... why is MOS 17 doing so well? I don't get that at all.
Yeah, I don't get it. #18 has the better cover, more stuff happens in the book and it's a full first appearance as opposed to a...fist. Smaller print run, perhaps? How many other characters have a cameo first appearance out-perform their first full and first cover?