The reason the PX100s are expensive now is because they have been discontinued, replaced by the PX100-II.
Personally, I like the PX100 and HD218 equally. Both headphones can be a bit too bassy with darker sources, though the PX100 here has the advantage of being open-backed, allowing some of that bass to escape. The HD218 on the other hand seems more dynamic, with better clarity in the midrange and treble, though its bass is a tad more boomy. Keep in mind though that the sound of the HD218 (and presumably its brother, the HD228) is highly dependent on fit; if the headphone isn't situated where drivers aren't pointed toward the ear canal, this could result in muddiness. Also, completely disregard the reviews on Amazon, Newegg, etc. that say that these lack bass. Really not sure how one can come to such a conclusion.
The HD218, despite its closed nature, does not isolate completely, as ljokerl has already pointed out. Expect no isolation with the PX100.
As much as I like them, I don't think either the PX100 or the HD218 are worth anywhere near their respective MSRPs. However, as you've stated, the street price of the HD218 is now sitting at around $30, which I think is a pretty good price, though you can get them for half as much if you don't mind refurbished units.