Showing Percent Discount on Cart Buttons

Last week we shared some wacky cart button shapes, based on Bryan Eisenberg's tip that unusual call-to-action buttons can improve conversion.

This week I came across a site that uses a very creative tactic with its cart button, which I have never seen before. ChristianBook.com includes the percentage discount right on the call-to-action button:

This is very persuasive on a product page:

Each product's button reflects the discount off list price, and is proudly displayed in category results:

And cross-sell suggestions:

This takes some serious custom programming, but it likely has a powerful impact on conversion. Are you taking notes, Amazon?