Well! It depends. On a lot of things. How long's left in your AT&T blood contract? Are you going to sell your receptionless glass-and-steel slab? Where do you plan to hawk your used wares? Trying to figure out all these variables could drive you crazier than your ex-iPhone's reception. So we did it for you.
The basic math
On AT&T, a two-year contract with unlimited talking, texting and 2GB of data runs $115 a month. (If you're grandfathered in with the old unlimited plan, it's $120.) The iPhone 4 is $200 or $300, depending on whether you want 16GB or 32GB.
On Verizon, unlimited talking, texting and data for an individual smartphone runs $120 a month. Phone pricing is the same with a new two-year contract.
Verizon, with unlimited data, costs $5 a month more than AT&T's current smartphone plans, which give you 2GB of data. On the other hand, if you've got unlimited AT&T, it's the same monthly price for all-you-can-eat. No savings or difference there.
The cost of breaking up
If you have time left on your AT&T contract—which you do if you bought a brand-new iPhone 4 for $200 last June—the early termination fee is $325, minus $10 for every completed month of service. So that's $255, if you bought that new iPhone when it came out seven or so months ago. Add in the 10 percent restocking fee (what a crock!) and you're talking about paying out $275-$285 to escape AT&T, depending on when you bought your phone. (Add $10 for each month after June that you purchased it.)
You could also try to get out of the fee altogether. Transfer your contract to some other sad soul. Complain, terribly forcefully and frequently—on record to customer service!—about the horrible service and hope they'll let you out without a penalty. Or you could move to Vermont, where AT&T basically doesn't exist, and argue you're out of their coverage area.
Finally, there's a one-time $35 activation fee when you walk in the door at Verizon.
Recouping your loss
You've made it out of AT&T alive. Did you manage to keep your iPhone 4? If so, you can recoup your losses pretty quickly by hawking that sucker. Verizon will give you $285 for a 32GB iPhone 4. You can possibly sell it for more at Gazelle.com—we saw an average price of over $400. And of course, there's eBay. Check out our sidebar for the latest trade-in values from multiple retailers all around the country.
The bottom line
How much is it gonna cost you, bottom line, no hardcore math hijinks to figure out? Well, like we said above, it depends. Here's some possible scenarios, if you've got an AT&T iPhone 4.
Scenario 1: You bought an iPhone 4 (we're using the 32GB model as our baseline in all of these scenarios, btw) in June 2010 with a new contract. You have completed seven full months of said contract, so you're already on the hook for $255. A new Verizon iPhone 4 costs $300, plus $35 to activate it. Trading in your old 32GB iPhone 4 to Verizon nets you $285. Total cost: $305
Scenario 2: You bought an iPhone 4 in June with a new contract, and have completed 7 full months of your contract. That costs $255. A new Verizon iPhone 4 costs $300 plus $35 to activate it. Selling your old iPhone on Gazelle nets you $420. Total cost: $170
Scenario 3: You got an iPhone 4, but don't have a contract with AT&T that you have to break. A new Verizon iPhone 4 costs you $300, plus $35 activation fee. Giving Verizon your old 32GB iPhone 4 nets you $285. Total cost: $50
Scenario 4: You got an iPhone 4, but don't have a contract with AT&T that you have to break. A new Verizon iPhone 4 costs you $300, plus $35 activation fee. Selling your old iPhone on Gazelle nets you $420. Total profit: $85
All in all, as long as you can sell your old iPhone 4 to cover the broken contract fee if you've got one to pay, the switch isn't terribly painful—a couple hundred bucks at most, or about as much as the phone costs in the first place. Besides, if you really need to go to Verizon from AT&T that badly, the sweet sound of reception is probably close to priceless anyway.