you know, the other way to think about it, is you actually get ALL of the space. But what are you going to do with an empty drive?
the drive needs some space to make a map, so the computer will be able to find files.
the OS puts a lot of files, some are vital, some are important to you, some you will never use. Most people don't have the ability to go through all the files to know what you need and what you don't. so there will be a fair amount used and some wasted here.
you also should always have 10% of your drive free for virtual memory. when you drop below 10% free you lose overall system performance.
then there is apps. some are installed by default, some you install some. some have features or languages you may or may not need.
All this adds up and no two computers are alike to know what you will have free after you start using them.
If your question is, how much does the default installation of OSX 10.6.x take up on a brand new computer, I think it is around 12gb.... but once you start using it, it is far more complex on how much space you have left for your data.