Well, let's take a logic guess.
If they release an iPad Mini it has to be cheaper than the 10" version.
That means $449 or less. Apple don't price after competitors, they focus on their own products. Soo:
iPad 10" 16GB: $499
iPod Touch 4" 32GB: $299
Where will it fit?
If they choose a retina display that means higher production costs and in my opinion they will go for the same resolution as the 10" iPad because they don't want the developers to develop for yet another resolution. It's one of Apple's big strengths compared to the competition that the developers can stick to a minimum of resolutions. Sometimes it needs adjustment(as with the bigger screen on the iPhone 5 and iPod 4"), but that's the natural development.
My guess is maybe a $299 8GB iPad Mini with retina display and the same processing power as the 10" iPad has now.
That $299 8gb retina would be good enough for me
But the 16gb is the default for new apple Idevices