Why is this book "worth" so much? Just because people like the cover? It makes no sense for a variant cover with manufactured rarity to be worth more than the character's first appearance. My , but what do I know?
It happens. A lot. But rare variant hunting is a different market than first appearance collecting. The ASM 678 VenoMJ goes for more than the first appearance of either character. The Wolverine/Deadpool cover goes for more than the first appearance of Deadpool, and even a Hulk 181 in every grade except 9.2 and up. The UXM 510 sketch goes for waaaaay more than the first appearance of Psylocke, and evidently even an X-Men #1 in low grades. You get the idea. In this case, X 23 is a relatively new creation and there are literally thousands of 9.8 slabs and 9.8 candidates out there in circulation, and let's face it, that X-23 Dell'Otto is probably the best depiction/cover of her that has ever been done to date and it has the additional benefit of being a "#1".
-J.
Kinda like Valiant books in the 90s