Adding to the above to provide a bit of sense of the scales of money here.
Allowing players to exchange real-world money for in-game money:
The current rate is (roughly) $35 -> 2x 30day PLEX -> 560 million isk. 560 million isk will get you 4-5 fully equipped and fitted battleships, or halfway to a equipped and fitted capital ship. Amusingly, as with any real-world currency conversion, exchange rates vary minute to minute, based on the current buy and sell orders on the market.
Allow players to buy their subscription using only in-game money:
It costs 280 million isk to buy 30d of game time. Operating efficiently in a profitable area, you can make about 25 million an hour hunting NPCs in 0.0 or running high level missions in empire. (You will need a character who's a year old or so to be able to fly the ships you need to use to do those things) So, all considered, you can play for free, if you're willing to put in about 16 hours of sweat equity per month. Of course, to get ahead in the game, you'll also need to pay expenses like ammunition, replacing lost or damaged ships, and you'll need to be growing wealth to buy more things in the future.