It’s $19.99 at Apple’s Mac App Store and it weighs in at 4.08 GB.
Update: Fall, 2017 – macOS High Sierra – Apple is now the current system, however if you have purchased Mountain Lion through the App Store, you should still be able to download it (through your Purchases tab).
It requires at least Mac OS X 10.6.8 or later and one of these Macs:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
- Xserve (Early 2009)
There maybe a retail version you can pick up in an Apple Store or have it shipped to you, but for now this is it, just like with OS X 10.7 last year.