No. You pay for the game, and you have access to all the single player stuff, and to the network as well. You can play on lots of servers and persistent worlds, or you can play in your house alone. You can also download lots of modules and custom stuff from here:
Neverwinter News, Trailers, Screenshots, Previews, Reviews, Guides -- Neverwinter Nights 2 Vault
That will expand the game play hours to years (provided you like the game, of course).
You can also act as a server, and play via ip with a friend or something if you don't like to play with unknown people but don't want to play alone either.
I recommend you to go the the Diamond Edition, because it ships the game + the two official expansions. Those not only give you more hours and extra clases, feats, spells, items and contents in general. But are also needed for most of the modules in the link I gave you above (since they use the new stuff in those two expansions).
Nowadays, it should be cheap enough. I suppose, somewhere in the 20-15$ range or so. Not sure.