A lot of DLC is content that is ready for the game and even some times left in the game, and all you do is buy an unlock key for the content. Othertimes, and developers have admitted this, content is planned to be ready for the game's release date, but then is cropped out not because it's not ready, but because it's "not needed." And then with this excuse, it's given to you as DLC later on.
A lot of the time it isn't brand new content that's being developed. It's old content that was dropped, already planned early on, or snippets of development that's already complete. And they get away charging you a good amount for such a small amount of content. Man, expansion packs have really died...