the whole plex issue itself is complicated.

Its basicaly CCPs attempt at a "FTP/Feemium" game. Originaly it sprang from a weird way to allow a small (they openly suspected around 5%) part of the player base to have an alternate way to pay to play by letting them pay ISK for GTC codes. then they realised that it was impacting the "black market" ISK sellers (who were offering a "better" return than the then existing GTC exchange) and created the P.L.Ex (Pilot Licence Extenstion)

Now theres one plex traded every month for every 3-4 accounts in game. more plex are/were created in 18 days in december of 2010 than are used in the previous 30(this was pre Aurum shop) this worried CCP enough that they actualy admited on more than one occasion that if they felt they needed to they would actualy step in and buy up "surplus" plex with ISK, or use plex to "sop up" excess ISK. (they have also started handing out Plex as a reward for signing people up to play. yes they know that some people will simply use it to spawn a small fleet of "51 day"alts, given the skill system in EVE that does not realy worry them)