Traditionally, the lower priced units retained a greater percentage of their resale value, while the more premium ones are relative money losers.
I suppose the rationale is that consumers who buy the higher end models would rather have the new models.
People who buy used or discontinued iPads tend to be more price rather than feature or value conscious, and would rather buy 'an iPad' as cheaply as possible, even though you could make an argument that a used 64 gb 3G iPad2 at $600 is a better value than a 16 gb wifi at $400.