So, on to PLEX and Aurum.
For me there are two possible uses for these cash-buyable things: training time and raw ISK. I sub my account in the normal fashion, so when I'm looking for training time I mean the Multiple Pilot Training Certificate. Let's start there.
Multiple Pilot Training Certificate1 PLEX will get me a month for $19.95 USD. Pretty straightforward.
3500 Aurum will get me a month, purchasable as a package of 4035 Aurum for $20.00 USD.
I'll wave fondly at the $0.05 USD and call those the same. So my money is better spent on Aurum since I'll end up with 535 Aurum left over. That's not even enough to get you a frigate skin, btw, so what would I do with that. Well, the obvious answer is that if I buy enough, and those ratios hold, then in seven purchases I'll get an extra month, right?
6 PLEX will run me $104.97 USD - in Aurum equivalents for the MPTC this is 21000 Aurum.
21200 Aurum will run me $100.00 USD.
Hmm, that 535 advantage for Aurum vanished. PLEX is discounted by 12.3% when you buy five of them, but for the same amount of Aurum you only get a discount of 4.8%
Raw ISKSo right now a PLEX is running at buy-side 1.188B and sell-side 1.200B ISK. That's the easy part.
Let's take a sample cool skin, the Astero Sanctuary skin. Everybody wants one, right?
The skin costs 1540 Aurum, so that's 2.62 of them per basic $20 package. Yes, I'm handwaving that fraction right now. The skin goes for 624 sell / 485 buy right now. On both the buy and sell side that's a better value than PLEX, though the range is a lot bigger between buy and sell and it assume you can make use of that fraction. If you just sold 2 instead of the 2.62 you would still be a hair ahead on the sell price at 1.248B ISK. You should expect that this might indicate that it won't be quite as predictable what final price you'll get after you battle the 0.01ers in Jita. But pretty clear win for Aurum, right?
Let's look back at those $100 packages.
PLEXx6 = buy 7.20B, sell 7.128B
Aurum = buy 8.59B, sell 6.68B
And that Aurum is actually you selling 13.76 Astero skins, so it assumes you can efficiently find a set of comparable skins that won't leave you with an inventory of unsellable Aurum. Granted, even if you only sold 13 Astero skins at sell price it would be better than the sell price of PLEX.
So it's an interesting world. For my simple original question of getting another month for my alt, the Aurum market looks interesting. I get what I want and have a bit of Aurum that eventually I'll use for something I'm sure. But if you're a speculator looking at the mass market, it's not quite as clear. If you're confident that you can get the sell price then you can get 119% of the ISK using Aurum. If you've got $100 you want to plunk into Eve, then I wish you the best of luck. I'm sure CCP is cheering you on too.