From the AWS blog:
"The Amazon Historical Pricing web service gives developers direct, programmatic access to over three years of actual sales data for books, music, videos, and DVDs sold by third-party sellers on Amazon.com. Third-party sellers can use this data to make pricing decisions based on historical prices and market trends.
Access to this data costs $499 per month for up to 20,000 requests, and $999 per month for up to 60,000 requests. All billing and service usage data is accessible through the “Your Web Services Account” button in the AWS Developer Portal."