The maximum value for an Apple iPad 2 64GB WiFi + 3G model appears to be $320. Here is a rundown of one can expect to get in regards to various models:
Apple iPad 2 16GB WiFi $205
Apple iPad 2 16GB WiFi + 3G $250
Apple iPad 2 32 GB WiFi $245
Apple iPad 2 32GB WiFi + 3G $280
Apple iPad 2 64GB WiFi $275
Apple iPad 2 64GB WiFi + 3G $320
These prices are based on iPads that are like-new - no functional issues, water damage, engraving, scratches, battery problems, missing power cords, etc. Problems affect the price Apple will pay, and the company also takes off for submitting a device that hasn't had its data wiped.
A few other outlets are offering cash for used iPads, with prices similar to the ones Apple offers - the only significant price difference is offered by Amazon, where a seller can get as much as $300 for a like-new iPad 2 16GB WiFi model, and as much as $460 for a 64GB WiFi + 3G model. Amazon, like Apple, also offers a gift card in return, which is good for users who just want to upgrade to the new iPad.
Apple also does trade-ins for the original iPad, starting at $110 for the 16GB WiFi model, up to $170 for the 64GB WiFi + 3G.
Hat tip to TNW.