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Amazon is offering Kindle e-reader owners an incentive to buy a new Kindle: trade in your old model for a gift card and a $20 credit toward a Kindle, Kindle Paperwhite, Kindle Voyage or Kindle for Kids bundle, which is currently being discounted to $89.99.

For certain older Kindles, it's not a bad deal. For instance, you can get $30 for 2010's Kindle Keyboard (formerly known as the 3rd-generation Kindle), which nets you $50 toward to a new Kindle if you factor in the $20 bonus credit.

Alas, Amazon is offering only $5 for the second-generation graphite Kindle DX, which cost $379 when it was released in 2010. You do get the $20 bonus credit toward a new Kindle, but even $25 isn't much for the jumbo Kindle with a 9.7-inch E-Ink display. On eBay, used Kindle DX e-readers are selling for more than $100.

See all of the news from Apple's March 21 event.