This has been thoroughly debunked in countless other threads. The iPad mini has a higher components cost than the 7" tablets.
The "X" models are needed for retina displays. Physical size of the display is irrelevant to the GPU. A retina mini with the same resolution as a regular retina iPad needs the same GPU power.
Looks like they are going ahead with the 28nm process after all. Testing it with the A5, it also now seems like they are going to apply that technique for the A5X which is mostly going to make the cut for the ipad mini.
According to my reading and research into the topic, I think they will utilise the A5X with a 28nm process, making it the
modified A5X ( or A6X for the mini)
Here's the potential benefit of switching to 28nm compared to the 45nm process they used in A5X.
1. Nearly 2x times higher speed
2. Upto 60% less active power consumption
3. 45% reduction in SoC size
Now considering tweaked Die-shrunk A5 chip as an example (2 means square):
1. A5 on 45nm -> 10.09 mm by 12.15 mm - 123 mm2
2. A5 on 32nm -> 8.19 mm by 8.68 mm - 71.1 mm2 (41% smaller)
3. A5 on 28nm -> 6.00 mm by 6.00 mm - 36 mm2 (50% smaller than 32nm)
Similarly A5X
1. A5X on 45nm process -> 163 mm2
2. A5X on 28nm process -> 81.5 mm2 (50% reduction)
Looks like there is substantial benefit switching to 28nm after all. Add this to an IGZO display, and utilizing the existing PowerVR SGX543MP4 or switching to the more powerful PowerVR SGX554MP4( from ipad 4), The new ipad mini with retina could basically be a generation old, but still be competent enough. It might also explain the slightly thicker back casing we saw in the pics on the rumour mills for the supposed mini retina.