iPhone 4, Droid Incredible, Evo 4G, and Nexus One hardware all cost about the same. But this chart shows exactly how much each will put you out of pocket—data/voice included—over the course of a two year contract.
The iPhone 4 fares well here, particularly in the Minimum Plan Total Cost category, but it's important to remember that AT&T is the only carrier currently offering tiered data. So that $2,000 is maxing you out at 200MB per month—not exactly a fair comparison to the unlimited plans found elsewhere. Even the "unlimited" plan caps you at 2GB, which is probably enough. But if it's not, you'll have to shell out $10 per 1GB per month extra.