Amazon's digital games section is worth keeping an eye on. Now and then the retailers drops the price of digital download games below what you'd find on the console.
CheapCollegeGamers.com points out that Amazon is running an Xbox Live Arcade sale right now, dropping the price on eight 80's classics from $5 to $2.96 a pop.
Included in the sale are:
Teenage Mutant Ninja Turtles