iTunes discounts a selection of metal MP3 albums. (The links are for reference; you'll need to log into iTunes on an iOS device and scroll down to the banner that says "Metal Albums from $4.99" to view the deals.) A couple of best bets:

• Metallica's ...And Justice for All MP3 Album for $4.99 (pictured, $4 low)
• Avenged Sevenfold's Avenged Sevenfold MP3 Album for $4.99 ($5 low)