boys boys, you're forgetting the fact that heart of the swarm is due to hit in very short time, with ad campaign already started, if anything they even bumped the price up recently just to get more money from the people that were waiting to play the humans campaign right before the next installment came out.
I'm not sure how much of an effect this has overall, but it does have an effect on current prices. (this has been proven with movies and books, when the next installement is about to come out the previous installments experience a surge in sales, which has caused retailers to crank up prices on existing installments when the sequel is right around the corner)