In the Spring 2016 RR Auction Space Auction this Neil Armstrong signed photo sold for $38,467 with buyer's premium. This is an amazing result... and to my knowledge is, by far, the most ever paid for a single signed astronaut photograph.
So, why so much?
In my opinion, this is the quintessential image of the space program... the moment Armstrong stepped off the ladder onto the moon's surface. And, it is signed perfectly with an Apollo 11 inscription.
There are only a few images of Neil Armstrong on the moon, and most were not readily available when he was freely signing autographs. Any lunar surface photo of Armstrong is uncommon.
Lastly, this photo came directly from the family of the former head of NASA's photo division, so it has strong provenance. In fact, an original of this photo would likely have only been available to someone inside NASA in the late 60s and early 70s.