The StarCraft II triology will be released in in the following way: First Terran, Second Zerg and then third Protoss.
What is interesting here is that the first game will be the base game and will feature a full campaign for the terran and a small protoss campaign along with it.
The other two games will be expansions which will add more content and will continue the epic storyline.
The price tag hasn't been stated yet but as a fan it doesn't matter.