Apple has stated that MacOS X developer tools will be free of charge, but you must register as a developer (which is also free of charge). If you sign up for one of the developer plans which costs money, you get monthly mailings, which includes seed software and releases of new operating systems (which you can then install on up to five machines). As a paying developer, the lowest cost is $500/year, but you also get access to Apple's developer tech support, and a few other odds and ends that most people don't need.