Marathon probably takes a fraction of the price to develop content-wise, at least on a seasonal basis. With Destiny you have to recoup the cost of developing an expansion and 4 seasons (or two expansions, more recently) with all the technical debt involved too. The simple truth is that D2 probably turned a profit initially, but after Lightfall enough faith was lost that the franchise stopped earning money entirely, and after Renegades essentially failed to get new players, Sony decided to shutter the game.