A rational agent should take profit, no matter how trivial, when no risk is involved. So yeah, it cost [ Microsoft ] nothing & they make money.
Nothing?
It cost them resources & time that could theoretically be used for more profitable ventures
That would be the case if they actually accepted bitcoin. (The tax reporting and auditing hassles alone would not make it worth the trouble.)
On the other hand, in order to accept dollars through Bitpay, they basically have to open a merchant's account with Bitpay, and insert some extra HTML (provided by BitPay) in their payment page. For accounting purposes, dolalrs received from BitPay are not different than any other dollar payments, and can just be added together with them.
Surely you're not thinking they're making any significant profit with Bitcoin as it is right now

They figured that bitcoiners are more likely than average people to spend money on video game points, so providing that option will make them more likely to do so.
The maximum amount of extra sales that they could get that way is 5 billion dollars, assuming that every bitcoiner will spend all his bitcoins on MS products, and the price does not crash. Needless to say, those 5 billion dollars would actually come from the pockets of those who buy those bitcoins at the exchanges. Actually there is no limit, since those coin buyers can use those same coins to buy XBox points too, and so on.
Of course not all bitcoiners will spend all their coins with MS. But even if only 1 of every 1000 coins out there is used once in 2015, that would be 5 million dollars of extra sales per year -- with practically zero investment.