The specific section on pricing in the api doc (https://affiliate-program.amazon.com/gp/advertising/api/deta...) seems to indicate historical pricing is allowed if you display the date with it:
"(o) You will include a date/time stamp adjacent to your display of pricing or availability information on your application if you obtain Product Advertising Content from a Data Feed, or if you call the Product Advertising API or refresh the Product Advertising Content displayed on your application less frequently than hourly. However, during the same day on which you requested and refreshed the pricing and availability information displayed on your application, you may omit the date portion of the stamp. Examples of acceptable messaging include: Amazon.com Price: $32.77 (as of 01/07/2008 14:11 PST - Details) Amazon.com Price: $32.77 (as of 14:11 PST - More info) Additionally, you must either include the following disclaimer adjacent to the pricing or availability information or provide it via a hyperlink, popup box, scripted popup, or other similar method: "Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on [relevant Amazon Site(s), as applicable] at the time of purchase will apply to the purchase of this product." In the above examples, "Details" and "More info" would provide a method for the end user to read the disclaimer."
Update: I'm thinking it's probably a violation of this:
"(n) You will not store or cache Product Advertising Content consisting of an image, but you may store a link to Product Advertising Content consisting of an image for up to 24 hours. You may store other Product Advertising Content that does not consist of images for caching purposes for up to 24 hours, but if you do so you must immediately thereafter refresh and re-display the Product Advertising Content by making a call to the Product Advertising API or retrieving a new Data Feed and refreshing the Product Advertising Content on your application immediately thereafter."