No it's absolutely free.
I've bought a few cash cards for fun and to give to people but you do not need them at all. In fact, there is a trading system in the game now so you can trade your game earned items for the nx (so if ppl are willing to trade, you can get this virtual cash, too!)
A number of us have been playing this for a while now --- the grinding is maybe a little bit more dull than other MMORPGS, but the cuteness of this game makes up for it
Seriously... SO CUTE. Maybe it's my Asian blood, but I love it.
I'm GelatoCrepe
Class: FirePoison Wizard
Level 63
Server: Windia
Right now is a good time to join, too, with the 3rd anniversary there's double exp and double drop (items) hours each day to help with the grinding. I play mainly at night after work, but if anyone joins on my server, I can help show you around.