You can pay whatever you want, some of it goes to charity, some of it goes toward my Ferrari.
If you're wondering why you should support things like this, here's an example: After constant harassing from John at Wolfire, I was able to get them Mac and Linux builds of Gish. Unfortunately we weren't able to test them too much (keep in mind we don't have huge QA teams, we just have to try and find a Mac or Linux user to try it out), so we've had some technical issues. Luckily most people have been cool about it, and most have been fixed, but I needed a new Mac to make an Intel version. Since the bundle is doing well I felt justified to finally get the new Mac you can see here: