If the price of your ticket is higher on the new day, you have to pay the penalty plus whatever the price of the ticket for that day...

Example:
If the original price of your ticket is QR 1000.. then you change your flight, and the ticket price for that day is QR 1500, you will pay the penalty plus QR 500. If the price for that day is lower than the purchase price or the same, you'll only pay the penalty..