Frankly I don't see the purpose of a 17" desktop, when you can get the same thing size screen in a much more portable laptop (as Chris has mentioned).
While cost maybe an issues to some, it has never seemed been the driver of Apples decisions. On the other have 'form' has, and even the lower cost items seem to have been driven by form over price.
The decision to produce the ipod shuffle and the Mac mini seem to have been driven as much as designing the smallest possible functioning music player and computer as by the desire for producing entry level products.
Will Apple Really Abandon the Low-End Desktop Computer Market?