GarageBand for Mac (v10.0) is available free on every new Mac and as a free download on the Mac App Store for all OS X Mavericks users. It comes with the full set of features and a great set of starter content including 50 sounds, 500 loops, 1 drummer, and 2 basic lessons for guitar and piano. With a one-time in-app purchase of $4.99 (USD), you can expand this content to 200 sounds, 2,000 loops, 15 drummers, and 40 basic lessons for guitar and piano. You also gain the ability to purchase additional Artist Lessons.
You can find this option by choosing Download More Sounds from the GarageBand menu. If you previously purchased this content, select Restore Purchase to install the content.