My advice is to not give up hope and keep checking up on eBay, and look at all listings for Phantasy Star Online, as it is sometimes mis-listed. You can always tell by the blue box. I managed to find a copy of Plus for £8 (roughly $12-13) just a couple of weeks ago. Keep at it, the bonuses are nice (not necessary by any means, but nice).
Official Scarlet Witch of the MvC3 Board. XBL GT: ChaosZero777