People are just catching on to the fact this comic has a 3 page preview for Invincible in it. That makes this book potentially a key issue long term.
A lot of stores will still have this in their back issue boxes for $5-10, maybe even their $1-2 boxes if the condition wasn't great.
It has already jumped to $100 for a high grade copy and who knows where it will go long term as more people realize they need this book to complete their collection of Invincible Comics.
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.