This is something we've all been wondering for a while and Matt Cutts, head of Google's Webspam team, kind of clears it up. The short answer is, about the same as a link.
The transcript of the video is below
Today's question comes from the United Kingdom. Sam Harries asks, roughly what percentage of page rank is lost through a 301 redirect?
Ah, great question, Sam. I've been meaning to talk about this for a long time. Let me give you the history on this. At some point, an SEO wrote to me privately and said, hey, I'm wondering how much page rank disappears when you have a 301 redirect. I wrote back and said, OK, why is that on your mind? And they said, well, the historical page rank papers always said that 10%, 15%, whatever amount of page rank gets lost, given any particular link. So from page A to page B, there's a link. The page rank that flows is, take the page rank and multiply by 0.85, 0.9, whatever it is, and then take the remaining links and divide that page rank equally between the outgoing links. So a certain amount of page rank dissipates on each link.
If 301s don't have that sort of dissipation, then instead of linking, I should do all my stuff with 301s, and I won't lose my page rank. And I'll have 10% more page rank, and more things will rank. And so you don't want people thinking along those lines, where they're going to shoot themselves in the foot trying to use 301s instead of links. And so at the next search conference, where it happened to be convenient, I mentioned that a certain amount of page rank also dissipates through 301s. And unfortunately, then the pendulum swung too far the other direction. People started to get worried, oh, if I use a 301, how much page rank do I lose? And so I sent an email to the team that is in charge of this, and, of course, the implementation can vary over time, but this has been roughly the same for quite awhile. The amount of page rank that dissipates through a 301 is almost exactly--is currently identical to the amount of page rank that dissipates through a link. So they are utterly the same in terms of the amount of page rank that dissipates going through a 301 versus a link. So that doesn't mean use a 301, It doesn't mean use a link, It means use whatever is best for your purposes, because you don't get to horde or conserve any more page rank if you use a 301, and likewise, it doesn't hurt you if you use a 301. So great chance to just sort of clear that up a little bit. It's exactly like having a link.
That's the current implementation. We don't promise that it will be that way for all time and eternity. But I don't see any reason why in particular it would change. So thank you for asking the question. Great chance to clear that up.