It will most likely be a paid expansion ~$30.00-40.00 like most MMO expansion packs. As far as content, there have been several times when the dev team said that 3.0 will be about as big as 2.0 was at launch content wise. So it will be pretty big. Not only in content, but we will also get new classes I'm sure we will see at least a new tank and healer type. Maybe even the much rumored "hybrid" classes.

We will also see a level cap increase, so expect to see at least 1/2 the new dungeon content aimed at level 50+, as well as a new endgame progression.