You can't export your song within Garageband with lower quality compression, but you can convert the song from within iTunes. Here's how:
First, export the song to iTunes
Second, select iTunes preferences, click Advanced and select the Importing tab. For "Import using" and "settings" pick your poison and click OK
Third, locate the song you just exported in the playlist, right click it and select Convert Selection to [the compression you just set in preferences]. For one button mouse diehards, select Advanced from the menu bar then Convert Selection to [the compression you just set in preferences]. iTunes will then create a new track for your compressed song, leaving the original intact.
Fourth, assuming that you will no longer need the original, right click it (you can verify if it's the original by selecting Get Info and checking the compression setting. It it's AIFF, it's the original), and select Clear. You should get a confirmation dialog. Click Remove. Another dialog should appear asking if you want to remove the original to the trash. You do.
All that should remain afterwards is a compressed version of your Garageband song.