As a Dad to a (former) 13-year-old I always made my son a deal when he asked for something: if you save/raise half the cash for whatever it is you want, I'll pay the other half.
He moaned like hell about it, but it helped him realise that, while he wanted something, he didn't actually need it.
Then one day, he surprised me! He came in to me and said that he wanted a PC!. Well you could have knocked me over with a feather! A PC!? When you could get a Mac? Get yer priorities right, Mush! But he gave me a carefully considered case for why he wanted it: gaming, but with the added bonus of being able to do schoolwork and teach himself web design, etc.
Well, to cut a long story short, he presented his argument well –*not just "I gotta have a PC and I gotta have it now!" – and he took me up on my offer! He went through all of his stuff and sold whatever he could, asked to do jobs around the house and garden, went out washing cars – and more.
I could see he was serious and that he was making a real effort to get what he wanted. It was different from everything else that he wanted, just 'cos his mates had one and he wanted to 'fit in', or that he had to have because it was the latest trendy gadget. I could have spent THOUSANDS if I'd pandered to his every whim.
But as he really made an effort and raised his bank balance from about £20 to £150, I could see that he was serious.
So I asked him for his £150, topped it up to the required amount and hey presto! he had his PC.
So show your folks that you are serious. Put a sensible reason to them why you need the MacBook. Sell your other Macs to raise some more money. SHOW them that this is important enough to you that you are prepared to put yourself out to get it!
I'm sure that they will help you out once they realise that you really mean it.
(Oh, and by the way, he's 20 now, lives in Sweden and STILL has that old PC!)