TBH, if it is bothering you this much, you might want to look at it another way. Does $15 a month seem worthwhile WITHOUT the story content? If the answer is yes, then simply view the expansion content as a monthly bonus. If no, then is the game PLUS the story content worth it? If the answer to this is yes, then you are fine. If the answer is indeed still no, then simply unsubscribe, wait until all of the chapters have been released and get it all for one month's sub.
I am also going to take a moment to predict a poodoo storm on the forums from a certain group of people complaining about people doing my last suggestion after they were subbed the whole time.