Paying for porting a number???

I have 3 contracts with Cell C, and my son has decided he wants to go with MTN as he got a cheaper deal. I then decided that my baby son could then continue this contract, and when I called I was shocked to hear that just to change the cell number (its a cell c prepaid) onto an existing contract I have to now pay R250. I think this is ridiculous as I am continuing the contract and its merely a prepaid number onto contract on the same network will cost me so much. I wish we were told about these things upfront