Are there instruments and other plug-ins available for download?

You may look at MainStage 2 at the AppStore. It is really not very expensive compared to original price of the GarageBand JamPacks, but includes all loops and Instruements that come with all the JamPacks. Installing MainStage 2 will make all those loops and instruments available in GarageBAnd and Logic.

DalilaB wrote:

Are there any free/low-prized plug-ins availble at all, and if so where?

along with Léonie's excellent suggestion of mainstage, there are thousands of instruments (SoundFonts), and dozens of effect plug-ins available online for free:

some SoundFonts (5,000 ish):

http://www.bulletsandbones.com/GB/PlugInsSoundFonts.html

how to install them (short story):

http://www.bulletsandbones.com/GB/GBFAQ.html#installsoundfonts

(Let the page FULLY load. The link to your answer is at the top of your screen)

how to use them (short story):

http://www.bulletsandbones.com/GB/GBFAQ.html#usesoundfonts

(Let the page FULLY load. The link to your answer is at the top of your screen)

install and use tutorial (the long hand-holding story):

http://www.bulletsandbones.com/GB/Tutorials.html#allaboutsoundfonts

(Let the page FULLY load. The link to your answer is at the top of your screen)

some effects:

http://www.bulletsandbones.com/GB/PluginsAUEffects.html

Install them (short story):

http://www.bulletsandbones.com/GB/GBFAQ.html#installaus

(Let the page FULLY load. The link to your answer is at the top of your screen)

The long story, install and use (including AU instruments):

http://www.bulletsandbones.com/GB/Tutorials.html#allaboutAUs

(Let the page FULLY load. The link to your answer is at the top of your screen)