With Microsoft, you can get their "Visual Studio Pro With MSDN Pro" subscription, which is currently about $800 a year. That provides operating system installs (which you have to unlock online, ten at a time), technical documentation, and the Visual Studio development system. I think you might get a few developer tech support incidents with that.
With Apple, you can join the ADC Online program for free; that gives you complete technical documentation and the XCode development system. Then you can buy Snow Leopard for $29 and do unlimited installs because there's no key - or if you buy a new Mac, you get it for free. You can don't get any developer tech support incidents, but you can join a boatload of free Apple-sponsored email lists, online developer forums, and so on which their internal engineers are pretty active.
In both cases, you still need hardware. For Mac development, that's $600 for a Mac Mini which includes the development system and unlimited OS installs. For Windows development, that's $800 for the OS installs and development system, plus whatever else you spend on hardware.
I pay the $800 a year for the MSDN/Visual Studio subscription. I also pay $499 a year for an Apple Select membership, which provides extra goodies like a discount on Apple hardware, beta versions of the OS, and developer tech support incidents.
MSDN was a pretty good deal a number of years ago, when they bundled Office with it. Now that it's mostly Visual Studio plus Windows installs, not so good. These days, I think Apple's offerings provide much more value for the money - especially the free ones - but YMMV.