Although I think first person shooters have been pretty much beating a dead horse ever since back around the time when they first started utilizing truly 3D engines, I must admit I will still play various incarnations of zombies, usually on BO. I like the social co-op nature of this part of the game.
To be honest, 90% of first person shooter games are all VERY similar. The entire genre is quite stale to be honest. The only thing that tends to vary at all is setting and polish. You can only head shoot a few thousand enemies before it becomes a boring routine. There are a few notable exceptions to this rule with titles such as the original Bio Shock or Gear of War where the feel, story, mechanics and setting of the game was fresh enough to breath new life into what is a dead genre for me, but generally when I pick up a shooter to try it out, if it doesn't grab me in the first 45 minutes then its not going to.
So to make a long story short, yeah its bogus the way a simple map is marketed as a $15 dollar upgrade to a $50 game but I think if you don't already have the old zombie maps and you like zombies that the Moon pack is a much better deal than most of the map packs in this series as far as how much content you get.
Still its a far cry from the sort of major content updates you see some smaller companies releasing for a fraction of the price. Look at Double Fine and Iron Brigade for example. The original game is 15 bucks, and the DL content includes an entire new campaign theater, dozens of radically new weapons and accessories and several new zombie style survival maps.(They even had an earlier completely FREE DLC that added a zombie style survival mode and new weapons prior to the major 5 dollar one.)
These big name companies need to learn something from the little mavericks companies who still show love for the fans by releasing either free or very friendly priced DLC packed with value and fun.