How do I convince my parents into letting me get an iPhone 4?

It's going to be my fourteenth birthday and I really want the iPhone 4. I have a phone right now but it's really crappy, and sometimes won't even turn on for days on end. Their only concern is the price. It's $200 for the phone itself, and then another $90 per month. My dad has the iPhone 4, and he can afford that, but he can't afford it for me. But I'm not going to give up because I REALLY want it. I'm about to get a job so I could pay off a little bit of the phone bill. But otherwise, do any of you have any ideas for convincing them into letting me get one?