I'm a little confused because she says it's too hard to keep track of two less expensive electronics so you are thinking of getting her one expensive electronic? Doesn't she carry a purse?
An iPhone 4 just doing a fast google search will cost you at least $200 with a new contract, depending on carrier. I do think making her pay half the data is a good idea because that crap is expensive and frankly if you don't have fancy phones she could be doubling your phone bill by herself!
I guess my two cents about it will show how I was brought up, but here are my two cents on phones for teens:
I'm 22. I'm not THAT old. I didn't start carrying a cell of my own full time until I left for college at 17, mainly because I started driving full-time then. I personally don't see what is so important about giving a kid their own phone if they aren't driving. It seems like an unnecessary expense and sets them up to be the kind of creepy phone/texting zombies I have to deal with every day. I borrowed an old family phone to keep in touch with for rides after school activities etc. That being said, "kids these days" all seem to want one. So I guess if it's what she really wants and you don't mind getting it for her as a gift, it's probably no different then the stacks of books I was doing at that age. But please don't let her delve off into the cyberworld and lose all face to face human contact! The zombies are recruiting...
My other point is, why an iPhone? Does she want it because it's an iPhone, or because it has some feature that she wants that a Droid or some other smartphone can't do? My point is I wanted an iPod and my dad gave me at 13 for Christmas a more reasonably priced competitor's model with superior features, and a little chat about how the "it" brand isn't always the best thing and it's important to shop around for any electronics purchase to make sure you get the best value for your money. That's a good attitude to have about electronics and I lesson I think more kids should be taught given how they're EVERYWHERE. But that's my personal soapbox, take or leave as you will.
I think your payment plan is fair though. A $200 investment for a phone now but $40 a month in perpetuity is an extravagant Christmas gift!
As for other gifts because now that I soapboxed I feel like I should give you some extras...she likes Legos...good girl!

I'm assuming if she's doing Legos at that age she might be into some of their Mindstorms stuff? It's making and programming robots, very age appropriate and a great outlet for
Lego kids coming to the end of their age range to have some good fun. Then there's the heavily suggested girly products - hair gels, nail polish, whatever she's into. If she likes books I can give you a big list of good stuff but I'll leave it off for now because it will make an already long post even longer!
Good luck with your decision!
