Free, but capped at lvl4.
Saying that you find gems that unlock the levels easily enough, for example I have an unlock for level 8 and I hardly played this at all.
All free unless you want to be drow, forged and another race I can't remember. Some maps etc need to to paid for too, but plenty are free and if you do want some content you only need to make a one off payment for the bits you want.
Plus it seem when you do buy anything the membership auto-unlocks to the next level, which gives you a few more goodies....see Cull's comments somewhere on this forum.