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.
Check out the documentation, code samples, and the WSDL file for more information.