I'm so frustrated! Why is it like this?

Ok long story short.. 8 months ago I ped my exams with good grades.. I told my mum I wanted a MacBook and she said yes.. She tried to make me change my mind so many times but I am ademant.. I want the MacBook.. She keeps on saying she will get it after she gets her bursary or after her pay check but it doesn't happen! I even offered her 200 quid of my own money to put towards it but today she said how about we go to Costco and buy you a Sony Vaio.. Mac's last for a very long time.. She knows it's a lot better but she's being a cheapskate.. Not only that she chose to pay for my cousins' Canadian school in Uganda.