[quote name='Jackovasaurus']I would think so. I've never seen a Mac app store gift card.[/QUOTE]
Your iTunes balance is shared across any type of purchase--iOS apps, Mac apps, music, movies, or TV shows.
They do sell App Store gift cards as well but it's the same thing, just with different artwork. You can easily redeem an App Store card and then buy nothing but music.
My experience has been that $80 for $100 worth of credit is the best available deal you'll be able to get at retail. I don't think you'll ever see better than $70 for $100; at that point Apple would be just straight-up losing money (70% of every sale goes to the studio/app developer).