Just checked around on my PSN just now and saw that Red Dead Undead Nightmare DLC Pack was 9.99. Not sure if it's discounted on XB Live as well but worth checking out.
Yeah, I think OP is a little confused, which is easy to do, since there's two DLC Undead Nightmare 'bundles', and one disc Undead Nightmare. I can see how one can get confused if you don't really pay attention to everything.
Thanks for looking out though, OP! Each of us has made a mistake before. No big deal!