The Nexus 9's folio keyboard case is an expensive accessory, even by Google's standards. The product, which both protects the tablets and supplies a Bluetooth keyboard, comes in at $129.99. But Amazon has recently dropped its price to $88, a difference of $42.
The case is already out of stock, but if you recently purchased one at its previous price, you can get Amazon to refund you the difference. Artem was able to get a refund despite pre-ordering one back in October, just by contacting customer support.