I've been meaning to pick up copies of Wars and From the Ashes (some of the few Greyhawk things I don't already own), but refuse to pay those high prices.
the Player's Secrets of Muden for Birthright is kind of rare, but I think copies of it only got for like $20-35 (haven't looked in awhile).
ECI were the ones behind the AD&D PBM book - that is, a set of rules for playing AD&D by mail. This 16-page book was approved by TSR and comes with Larry Elmore cover art. The whole series is probably not worth your attention - every gamer I asked about these (and I have seen a few of the books myself) told me they were rather useless.