In the CSM9 Winter Summit there was a discussion on Plex which perhaps deserves more attention for those holding and trading in Plex. The relevant part is below and my 6 bullet point summary is below that:
"The next topic was PLEX prices. The price has dropped recently from recent high prices (around the 1
billion ISK mark) to around 800mil, but it did dip down to 700-600 mil. CCP Thomas also showed some
statistics related to customers using PLEX for subscription. There were some discussion about whether
CCP is manipulating the market at present and what their metrics are in that regard. There were some
questions regarding player manipulation of the PLEX market, which CCP confirmed as happening just like
any other item in game.
There were some questions about the conversion rates following "This Is EVE".
There were more questions about PLEX prices and whether there was a target price. CCP Thomas
explained that there wasn't a set price they were targeting, and it was more about the delta in prices.
There was discussion about player reactions while PLEX was around 1 billion ISK, and the actual number
of players using PLEX was relatively stable.
There were also some questions about the number of PLEX used for other services, especially multiple
character training. At this point, CCP Foxfour also explained that they wanted to move away from people
directly buying PLEX and instead move to people just buy AUR. This AUR could then instead just be used
to buy PLEX from the NES store along with everything that is currently using PLEX, moving from a three
currency system to a two currency system. There were questions about whether it would be possible to
directly trade AUR for ISK on the market. CCP Cognac responded that there are no plans to do so. Sugar
Kyle brought up the topic of sending AUR to other players. It was explained that while it would be
desirable in some contexts (such as gifting someone else some AUR) there are some issues, including
some legal ones that need to be covered."
Source: CSM9 Winter Summit Forum Post
The summary of the above is:
1) CCP knows how much of the Plex is being used for Subscriptions vs Other Services vs holding for investment
2) CCP knows / believes the Plex price is being manipulated by some players
3) CCP is more concerned with the change in the price of Plex, not the level
4) When Plex hit 1bn the number of players using Plex remained stable
5) And, the interesting part, CCP may want to move away from Plex for US$ towards Aurum only. This Aurum would then be used to buy Plex as normal
6) No plans to trade Aurum on the market for ISK
The key points of interest are (5) and (6)
Firstly, the bad news is that CCP may break the direct link between US$ and Plex by gettung Aurum in the way.
Secondly, the good news is that Plex will still be the way to extend Play Time and for Other Services.
But, the route to Plex could be interrupted by a Player stopping at the Aurum store and spending the purchased Aurum on something else instead of Plex. In other words, the change CCP is thinking about can only reduce the supply of Plex (or leave it as before, at best).
That, all things being equal, would push up the price of Plex.
However, all things are not equal. Aurum can already be used in the place of Plex (and the other way round).
In Jita prices (1st Feb 2015):
It costs 3500 Aurum to do Dual Training vs 1 Plex at 800m ISK. That implies 1 Aurum is valued at 0.23m ISK.
1000 Aurum is selling for 500m ISK and buying for 200m ISK (not a very liquid market - hardly trades at all) which implies 1 Aurum has a spread of 0.20m - 0.50m
It I assumed the Buy orders for Aurum were fair, then the current Aurum / Plex price is about right.
So far.
(edit: edited the price of aurum to reflect i missed the "m", i.e. 0.23m ISK not 0.23 ISK per aurum)