How can i convince my parents to let me buy an iPhone 4?

[Jerome]

Ive been waiting and saving up for it for a long time now... Ive saved up $336 for an iPhone 4 (16gig= $199) , and now that i have enough money, my parents wont let me buy it because of the extra $15 for the data plan. I get $30 a week ($120 a month). I told them so many times now that i can afford to give them an extra $25 just to help them pay for the data plan and plus, im paying for the actual phone itself! I dont understand why they wont let me. Ive been asking them if i can buy an iPhone 4 since the day of its release. This really sucks and im pretty sure im responsible enough to handle that phone. I always make my parents happy by getting almost all A's in my report card, but they still won't get my an iPhone for some unknown reason. And now they get mad everything i ask them. I'm 15 and its the only thing i want!