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MattFrank - Tim is working on an article on the topic that will have a lot of information, but here is a brief answer.
It seems Sporting Life switched to the issuance of the "300 Subjects" back when series 19-24 were released. Cards in those series should only exist with that back. They also re-issued some of the earlier series cards form that point onwards and those re-issues also contained the "over 300" back.
Much of the issue of scarcity has to do with how they were distributed, in series. If you already bought a Honus Wagner during the first several months of release, you probably wouldn't buy another, and therefore the re-issued early series cards with the 300 backs are somewhat difficult. That said, series 1 was overloaded with stars and certainly more people bought series one cards even later in the run, then bought cards from other series due to the star quotient. Therefore "over 300" backed cards of players not in the first series, and players not in series 19-24 are pretty tough. Cards in series 19-24 are more scarce then their earlier counterparts due to a more limited time of being avialable, but since they only contained new players that had not yet been released, buyers who were collecting the whole set, bought them as well.