paying 5 time the initial price

i arrived at OR Thambo international airport 40 munites before my flight when i checked in i was told i missed my flight already, i accepted that. They told me i should go and reschedule my flight. when i arrived at help desk i was told i need to pay an extra 3800 for a flight from OR Thambo (Johannesburg) to King Shaka (Durban). Mind you, i bought the initial tickets for R600 per person which was R1200 for both me and my partner. This means a ticket jumped from R600 to R2500.00 which i find unacceptable. Our next flight was suppose to departure at 15h20 but we ended up deputuring at 16h15, mind you we were 40 minute early for our initial flight