Change of flights cost a fortune

My boyfriend recently booked a flight on Kulula from Durban to Cape Town. After a change of plans, he had to change the destination of his flight to then fly from Durban to Port Elizabeth. Kulula don't fly this route and so his only option was to change this flight to a British Airways flight. This flight on the website cost R1000, the same price as what he had paid for the original flight so he assumed that he would only be charged to change the flight. When it came to changing it, he was told that he needed to pay an additional R1000 for taxes as a result of the change in airline. I feel that when convenient for them Kulula and BA stand as one but when it comes to things such as this, they are looking to make an extra buck. As a regular customer, this is very dissapointing.