Pay attention to changes in the Steam store page!
It’s all about what you have asked us for:
1) The base game version now includes a collection of missions for a singleplayer experience (5 missions as a prologue to each of the main campaigns)
2) Package with two campaigns was replaced by a “Classic” version, which includes 4 campaigns and a Skirmish package
http://store.steampowered.com/bundle/590/
3) DLC campaigns are now cheaper!
https://www.youtube.com/watch?v=C6Px3IoeU5E