The value of 181 will continue to rise so long as Wolverine cements himself (via movies, video games, etc.) a place in US pop culture, the way Batman, Hulk, etc did decades before. A problem with Wolverine is that he's newer than the classic characters, so there's still generations of people around today who did not grow up with him (how many of us had/have an older family member who collected comics as a kid and talk about Fantastic Four or The Flash but wouldn't know Wolverine if he smacked them in the face), and of course Hulk 181 is not all that rare. But he's probably one of the last mega-stars to come out of Marvel or DC, so I don't see the issue ever slowing down completely, IMO.