What happens to authors' commissions with a sale like this? It says on the site that authors get 50%. Do they continue getting 50%, just of a smaller pie? Or do

What happens to authors' commissions with a sale like this? It says on the site that authors get 50%. Do they continue getting 50%, just of a smaller pie? Or do they receive the same amount per book sold and PragProg eats the discount? If the former, do you have to get permission from the authors to do the discount or is it baked into the contract when they sign with you? I have no idea how the publishing industry works and I'm just curious. And thanks for the discount!