Think I found the answer. Exclusivity has a value. Just the formula for calculating income is now this:
The item list price is $49, your author fee is 30% (12.5%-37.5% - Excl.), and the fixed buyer fee is $12.
$49 - $12 = $37
(List price) - (Fixed buyer fee) = (earnings without author fee).
Earnings without author fee - % author fee = % Earnings
is that right?