In the most recent Bungie Weekly Update released July 27, 2014; Bungie staple, Urk, took over writing duties for current community manager Deej and made it clear once and for all that the developmental costs for Bungie’s adventurous new I.P. “Destiny”, are “Nowhere near” the previously touted $500 million mark.
In his response to a fan question asking if "Destiny" was the most expensive game ever developed, Urk made it a point to quote Bungie Chief Operating Officer, Pete Parsons and his comments in a recent interview given to GamesIndustry.biz in which Pete stated that “For marketing you'd have to ask Activision people, but for development costs, not anything close to $500 million. I think that speaks a lot more to the long-term investment that we're making in the future of the product.”
Deej also made it a point to add “But the budget for Destiny, including associated marketing costs and pizza Wednesdays, is nowhere near 500 million dollars.”
The initial $500 million amount seems to have stemmed from a May 6th article by Reuters which reported on the Milken conference held in late April 2014.
During this conference, Activision Blizzard CEO Bobby Kotick, stated that "If you're making a $500 million bet you can't take that chance with someone else's IP. The stakes for us are getting bigger."
It would appear as though people then ran with that quote and saw it as though Activision was dishing out a whopping $500 million purely for developmental costs when that is not at all the case.
From the recent clarification in the July 27th Bungie Weekly update, it would appear as though the $500 million is more of an investment into Destiny as an entire franchise rather than just the first game in the series.