I certainly don't know enough to accuse anyone of tax evasion. But it does seem to be a strong incentive to classify lots of different activities as R&D.
Let's assume that Microsoft started work on the console in 2007, two years after the Xbox 360 launched. Our $100 million has now become $16 million per year. Developing the controller is going to involve a lot of people: software, hardware, QA, design, supply chain, fabrication, etc.
Let's say that Microsoft has a team of a hundred people working on the controller (personally I think that's a very conservative guess). So of our $16 million we've easily eaten up half of that in labour costs. Prototyping, gearing and tooling up, and all the various other expenses associated with designing and manufacturing electronics eats the rest up pretty quickly.
This is obviously 'back of the napkin', hugely simplified conjecture, but I can definitely see how $100 million becomes a fairly reasonable figure over time.
[xbox one vs. 360 controller](http://assets1.ignimgs.com/2013/05/22/img6866jpg-5ea6bc_610w...)
With that said - they did invest in "smell-o-vision", having other displays, and probably a few other far-fetched technologies they haven't discussed. But $100 million is still a lot of cash if you ask me.