AT&T charges $20 a month for unlimited texting. That's $240 a year. In the two years of your contract, $480. Free texting would "pay" for the iPhone twice over.
I have unlimited in my Sprint plan and I pay $55 month with 4G service on my Evo.
It looks Sprint charges around $10 a month for unlimited messaging (the basic plans are $10 cheaper than the basic + unlimited messaging). Though you can also add unlimited messaging to a basic plan for $20... Odd.