It's hard to get a good bead on exact numbers. As was mentioned earlier on the Boards, Gerber did a good job getting the relative scarcity right. That is, typically 9's are scarcer than 8's, 8's scarcer than 7's, and so forth. Of course there are mistakes as noted by walclark above. As pointed out by a few boardies, however, the scarcity values do not reflect the underlying numbers that Gerber suggested. He almost always underestimated the amount of existing books.
What I find funny is how we GA folks talk about books not being that rare when there might be 45 copies as opposed to 15 suggested by Gerber. When, on the other hand, SA folks make statements about how rare some issues are with fewer than 2000 copies like Tales to Astonish 27 or Journey into Mystery 83. Our rare is still a magnitude or two rarer than the SA metric :-)