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You can actually see what discount you get in the US. Just go to the educational store portion of Apple's website.

Example:
baseline MBP 15", $1,799 original, $1,699 with educational discount (About 5.5% off)
high end MBP 15", $2,199 original, $1,999 with discount (About 9.1% off)

etc etc etc

Looking at the prices, looks like if it the original cost is cheaper, then less discount.
 
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