In one of the astronaut collecting listgroups, a member theorized that space autographs will get even more valuable over time as space exploration advances. Here are my thoughts:
I think there will always be a "bump" when current events warrant increased attention, and a Mars mission (and maybe even the Chinese mission) will draw huge interest to all space pioneers.
Someone used the Columbus and Charles Lindberg example with which I agree. But who remembers the 2nd guy to cross the Atlantic solo and when is the last time you thought to look for his autograph?
In the long run, I feel that only a handful of space pioneers will continue to draw interest except for the most devoted die-hards. In 50 years, there be interest in Gagarin, Shepard, and Armstrong. But I'm afraid that Alan Bean, Scott Carpenter, Owen Garriott, et al. will settle under the dust of time and history. Most of our archivally stored SPs will end up in estate sales or on dealer tables next to the shoebox full of silent film star postcards.
Hopefully I'll still be around to prove myself correct.