Even though Buzz Aldrin is a common signature, his signing fee is high (around $400 last time I checked). Apparently, he still gets people to pay the fee. A common signed Aldrin item might command $100 - $150 on ebay.
Mike Collins used to be the most uncommon signature among the Apollo 11 crew. He signed freely for a few years after Apollo 11 until the mid-70s when he worked at the Smithsonian. After that, he became more reclusive; he didn't hide his dislike for autograph requests. In recent years, he has conducted private signings and appeared at a few signing events. Still, his signature is in high demand.
A signed Apollo 11 crew photo is one of the holy grails of astronaut autograph collecting, commanding thousands of dollars. While I'd love to have one in my collection, I think there is a higher probability of me being the next man to walk on the moon!