I just read my Tidbits email:
"a TidBITS reader pointed out that you could pay more or less for the same configuration, depending on how you started.
In short, for certain configurations, starting with the high-end options will give you a better Radeon Pro graphics processor than if you start with a less-capable configuration and beef it up to be otherwise identical.
The mid-range line starts with a CPU that retails for around $259.
The high-range line starts with a CPU that retails for around $279.
To upgrade the mid-range line to the CPU spec'd above, they want $500.
To upgrade the high-range line to the CPU spec'd above, they want $400.
The upgraded CPU is $549 retail.
There is around a $50-$100 price retail price difference between the video card configurations, with the more expensive being the higher one included in the high-range.
If the mid-range CPU upgrade price was $400, to match the upgrade price to the same chip on the high-range, then you would have a $100 price difference which would be explained by the graphics card differences. But that $500 upgrade charge seems a little suspicious.