Doesn't World of Warcraft have a subscription model? Are you considering those costs too?
Comparing each expansion release individually to the cost I paid for it isn't working in their favor, and only makes it harder for me to justify "the next expansion" if the perceived $/content is too low for the last couple. As opposed to the aggregate model I might use with a subscription service where maybe I didn't get all of that year's cost value in any single month, but cumulatively it was great.
Anyway, part of the driver of the current business model was supposedly Activision's fault in trying to CoD-ify the franchise, so maybe Bungie can course correct here somehow. (I'm a bit doubtful, personally, but will be watching.)