The reason you can't find a reference is because MS doesn't make those numbers public, which is also why my estimate is so vague.
Anyway, for Microsoft's
FY13 Q4 their EDD (entertainment and devices division) reported the following:
"Research and development expenses increased $432 million or 28%, reflecting $246 million higher headcount-related expenses, resulting mainly from increased headcount in connection with the Xbox platform and Skype."
So, that reveals that for 2013 MS' EDD had R&D expenses of roughly 1.974 billion (for FY13 MS' total operating expenses were around 30 billion). How that is distributed across the three groups is anyone's guess, but a good chunk of it will be going towards WP, as they are the only group actually developing an OS. So, just R&D is already at least a 100 million per quarter, likely more. That doesn't yet include WP's marketing expenses, which can themselves easily reach 200 million per quarter (for FY13 MS spent 15 billion on sales & marketing).
None of this yet considers the affects of the Nokia acquisition. I'm not sure how that factors in, but it appears MS is only keeping that part of Nokia which ran a deficit, so that will add even more expenses, but I have no idea to what extent.
The result of all this is a number as vague as "a few hundred million".
BTW:
In 2013 WP wasn't yet free, so it generated revenues. These are estimated to have been around 0.8 billion (
this is the best breakdown I've seen so far). But this doesn't factor into this calculation, as it was completely offset by the billion MS paid to Nokia for their "joint strategic initiatives".
In 2014 WP is now free, so those 0.8 billion are lost, but MS is also no longer required to make those payments to Nokia. That roughly cancels out, so MS is left "only" with the R&D and marketing expenses + the expenses for Nokia.