I got both the first and second game on sale, as I did nearly all the DLC, so I have no idea how much I paid. TWW3 and it's DLC I bought at full price.
So far as I know, all the factions are present. This new $25 DLC is a pack of 3 lords and a hero for existing factions.
The Immortal Empires campaign is the one nearly everyone plays. It's just a map-wide free-for-all. Nobody (but me) liked the campaign that launched with the game. Too many people failed the campaign, and people don't like losing. It's been softened up substantially from the way it was at launch.
The Immortal Empires lets you play as any faction, so it has a ton of replayability. The campaign that came with the game restricts you to just a few factions.