The always quote worthy Activision president Robert Kotick gave a ballpark figure of what Activision are investing in Destiny at Milken Conference in Los Angeles last week (via Reuters).
"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."
A PR rep later elaborated to Reuters that this number included "marketing, packaging, infrastructure support, royalties and other costs" - some of that will naturally flow into sequels (specifically the infrastructure bit). After all Activision have a 10 year deal with Bungie on Destiny.
Nevertheless, the number is astounding as the most expensive game to date Grand Theft Auto V is estimated to have set Rockstar back $260 million.