IT COST MORE TO CHANGE A FLIGHT THEN I PAID FOR IT

My sister has recently immigrated to Australia and asked me to babysit my niece over December / January. I said that's fine but it was already a huge inconvenience for me as my boyfriend is moving from East London to Johannesburg and his flying up on the 27 November so we can drive down together on the 12 December to fetch his stuff, we were meant to drive back together end of December. My sister tells me then that I must pay for the ticket to and from East London to Johannesburg for the flight to Adelaide in Australia. I bought these in September as to get a good price. Now my sister has booked me a ticket returning on the 12 January and my ticket to East London is booked on the 13 January, to change my ticket will cost me more than I paid for the ticket. I don't understand what it is I am paying for? I am doing the change via the manage booking online and the price is the same price as the initial ticket so I'm paying R300 admin fee? Who is doing the admin if I am doing it myself online. I am usually so happy with Kulula's service for this time has really let me down as my boyfriend and I are using all our money for this move and now this.