Temporarily out of stock, but can still be purchased at the discounted price. Select Amazon.com under "More Buying Choices" in the right sidebar.
Amazon has Batman: Arkham City Game of the Year Edition (Xbox 360) for $20
with free shipping on $25+. This edition includes Harley Quinn's Revenge which contains over 2 hours of content, plus Catwoman Pack, Robin Pack, Nightwing Pack, Arkham City Skins Pack and Challenge Map Pack.