Affiliates & ProSites coupons

if a coupon is offered for a pro site, what is used to calculate the earnings for the affiliate ?
is it based on the cost of the pro site or the amount actually paid ?
if using a fixed $ amount for the affiliate i cannot see this being an issue but what if % is chosen in ProSites settings for affiliate payment ?

for example :
$20 for a pro site.
coupon is %50.
affiliate is entitled to %50.

new site signs up through affiliate link and uses the %50 coupon and therefore pays $10.
so does the affiliate earn %50 of $20, or %50 of $10 ?
does the affiliate earn $10 or $5 ?

Dam my math is rocking tonight !