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.