The game's subscription model is actually implemented in such a way that you can have an item (a "Pilot's License Extension", PLEX) that represents a month of account time. PLEX can then be bought and sold by players in-game, at whatever price they're willing to agree to. There's a fairly active market for them, so a USD -> ISK (ingame currency) conversion ratio can be calculated.

Of course, there's no (officially sanctioned) way to convert PLEX back into USD, so such comparisons speak more to CCP's profits than anything else.