Who says that they don't invest in own studios since a while? But as long as the userbase of the new gen is still so small it isn't that expensive to get third party exclusives.
It also makes no sense to have too many own studios since releasing too many games does only kill sales of every single game. I think Microsoft will have a pretty healthy amount of own games this gen. More would be always great for us players but It has to make sense for the company to have thousands of employees working on games and if they release so many games that people have to decide which one they can buy it will simply cost too much to pay so many salaries.
And not every third party exclusivity has to be that expensive. If you buy a game which would also come to other consoles it is but if you give a developer money to make a game for you (like Killer Instinct) it isn't that expensive and makes as much sense as having own studios. Or do you think it would have been so much cheaper for Microsoft to open an own studio to work on Killer Instinct? And Dead Rising was probably also not much more expensive as if they would have to have an own studio for it.
And the Titanfall Bundle doesn't have a retail copy, it has a digital code.
Phil Spencer also explained once in a Podcast that it isn't so simple to say that it is so much cheaper to have more own studios as to pay some third party developers from time to time. People really underestimate what it costs to have own employees. It's not only the salaries you have to pay. Why do you think Nintendo doesn't simply open 10 more studios to compensate all the thrid party games they miss?
Your calculation lacks a lot of factors so that I don't even try to argue about that. Not to forget that you simply can't do a calculation like that in 5 minutes or even 5 days.
