Since the iPhone 3gs Apple always had a cheaper sub-version of the latest and greatest in a product line. For the iPhone that was the 3gs when the iPhone 4 came out. The iPhone 4 was the cheaper version of the 4s. And the 4s again is currently the cheap version of the iPhone 5.
The same goes for the iPad line. When the iPad 2 came out you could still buy the iPad 1 as a cheaper version. And after that the iPad 2 became the cheaper version of the iPad 3. But today something has changed which I still do no not understand very well. The iPad 4 is a minor update to the iPad 3 so it is understandable that the iPad 3 is not the cheaper version of the iPad 4. Apple basically killed the iPad 3 and still leaves the iPad 2 as a cheaper version of the 4.
However… the iPad mini also got introduced this week and despite of the iPad Mini hardware being developed from the ground up it is still a shrunken iPad 2. Although the screen size is 40% smaller it has the same features and performance as the iPad 2. I question why Apple didn’t also kill the iPad 2 and let the iPad mini be the cheaper version of the iPad 4.
The only reason I can think of is that the smaller screen size fits a new market of which Apple is unsure how successful it will be. Besides screen-size the two are fully redundant, even in price. There is an advantage even in killing the iPad 2, because the Mini sports Lightning connector and if we look at the update of the iPad 4 they are really pushing this. Therefore I won’t be surprised if Apple kills the iPad 2 in the nearby future.