Microsoft Windows Maintenance Cost?

Microsoft is maintaining so many version of Windows Operating system since many years; do they also bear 60% of the software development cost in maintaining these OS? as far as i know, i only occasionally receive few software security updates from Microsoft Windows,  i install the pack and that is it; i don't pay any additional cost for the security package upgrade to my existing Windows 7 OS.  kindly explain?

 You are recieving updates or security alert then you have registered operating system. Cost is paid by registered user and he/she always recieves updates. In Pakistan, copy of registered windows are sold as crack version which is used by a large number of users. Otherwise, when you buy registered window, you have paid all cost and recieve updates for existing window.