Microsoft hasn't disclosed how much it charges phone makers per device for the Windows Phone OS. I've seen reports of anywhere from $5 to more than $30 per phone. The price is likely dependent on volume of phones built/sold.
Meanwhile, another March 13 report -- this one in TrustedReviews.com -- quotes an official with Huawei, another Microsoft Windows Phone partner, as saying Huawei will bring a dual-boot Windows Phone/Android device to market this year. This isn't, as some have believed, a phone which would provide carriers and/or users with a one-time choice between the two operating systems. Instead, it would be a device that would allow users to switch at will between the two operating systems.
Microsoft is relaxing its licensing terms and conditions in order to attract new Windows Phone partners even though it is preparing to offer its own phones once its acquisition of Nokia's handset business is approved by antitrust officials around the globe and Microsoft takes ownership. The company has removed certain requirements, such as dedicated capacitive buttons on Windows Phone handsets, so OEMs can opt to put Android or Windows Phone OS on the same device to allow those OEMs to cut costs.
I, myself, would think offering users a choice of OSes on a single phone as Huawei is seemingly planning would be more confusing than anything else. It's hard to see why users would use the Windows Phone OS at all if Huawei is planning to offer the full Android/Google stack, plus all the apps in the Google Play Store. It's not clear from the Trusted Reviews post if that's definitely part of Huawei's plan, however.
Microsoft is counting on continuing to target the lower end of the phone market as the one where it has the biggest potential for volume growth with Windows Phone. Windows Phone's market share is still hovering around four percent globally.