Changing of my Ticket

I have a flight booked from PE to JHB on the 3 August, which i payed R593 for. I need to change my flight to November. When i went online to change the ticket to the new date (7 November) the cost of the ticket is now R877, plus i would need to pay in a penalty of R300, which i am full aware of.... I double checked the price for the same date and time on [URL Removed] and the ticket price was R494?? I called Mango to ask how can the ticket prices be different? So now to change my ticket i need to pay a total of R1177 for my ticket that should only cost R493 plus R300 penalty (Total R793).... No other airline does this as i changed my other flight with flysaair, where the ticket costr was the same all i had to do was pay in the penalty! Mango is ****************! So if i change my flight i will know that every other person on the flight just payed R493 for their flight and i payed R877 pl,us R300 penalty for my flight. Why the **** must i pay more and still pay a penalty for my flight??? I am seriously not happy with Mango and their service!! I fly at least once a month with Mango and then i get this kind of service!! This is ********!! I dont have this problem with the others