We comic folk are odd beasts for sure. I was going to roll my eyes and scoff at the idea of plunking down $100 for a comic with a three-page preview of a different book in it. Then I remembered I paid a fairly significant amount just to get all the variant covers for Gen13 #1, essentially buying the same story over a dozen times for a different piece of cover art each time, so... :D
It's all about what was the first time a character, team, costume, or new person becomes a character. No different then card people buying rookie cards.
People love to collect and no matter what it is they collect "first of" items almost always are the most coveted.