even if you buy single (to go) and single (to come back), it won't change the price. but you know how airasia flight rates always change right? like it could be cheap today, and expensive tomorrow. so obviously if you buy the plane ticket earlier, or during promo, it will be cheaper lah.

to cut it short, buying one way or return is the SAME price. WHEN you actually buy the ticket is what makes the price difference.