Fortunately, reader Andrew has come up with an incredibly awesome iPhone plan calculator. Simply slide the scales to your approximate usage, and you'll be given a solid guide of which plans will suit you in your iPhone purchase, including the monthly cost and the total cost of ownership.
If Pythagoras was alive today, this is exactly the kind of practical mathematics he would spend his time working on. If you're looking for an iPhone, make sure you check this out before you empty your wallet.
[Numbers in a flash - Thanks Andrew!]