They already had the big fat check option when they were at Microsoft and central to their strategy. I don't think it's a money issue as Microsft would have paid whatever they would pay now back then, probably more as Bungie were untarnished at that point. I got the feeling Bungie wanted to do their own thing and be able to stop the intense focus on doing Halos and release whatever they wanted to the whole gamer audience (rather than just one set of fans). Plus Microsoft have enough cash to buy nearly all the game developers outright but they rarely do. Winning a console race doesn't necessarily mean good profitability or good value to shareholders, especially as you lose a big part of the asset's value (the price and excess you're paying for) when you turn a studio or game exclusive.