DDO has three tiers: F2P (Free to Play), Premium, and VIP. VIP is a subscriber -- you pay IIRC $15 a month, and have access to all content. F2P is just that -- you had paid nothing (as of yet), have access only to free content, and can buy any premium item separately.
Premium is what F2P becomes once you buy anything at all. There is significant difference between F2P and Premium, so for as little as $6 (once, not per month) you can make your game a lot more enjoyable.
Price is a bit difficult to figure out because you do not pay for things directly -- rather you buy "Turbine points", then spend them on things like premium adventure packs (so far I bought two). Turbine points are not only bought with money but also acquired by achieving certain milestones within the game; sometimes they are on sale. Often adventure packs, and other things you buy with Turbine points are on sale. So far I paid $16.50 over five months. It will be a while before I buy any more Turbine points, as there is so much free content to explore. Heck, I am not even done yet with one of two premium packs I bought!