iPhone users sick of AT&T's network and hoping that Verizon's will be better will have to pay a hefty price if they want to try and switch.
First you'll have to break your contract. If you bought after June 1 2010, when AT&T hiked their Early Termination Fee (ETF), that means a $325 ETF, minus $10 for every month you've had the phone. Those who bought before June 1, 2010, should prorate based on a $175 ETF. This calculator will do the math for you.
Then you'll have to sign a new two-year contract with Verizon and buy a new phone, starting at $199 for a 16 GB or $299.99 for a 32 GB. Nope, you can't transfer over your iPhone from AT&T. Built-in lockouts aside, AT&T's network is GSM and Verizon's is CDMA. You can't use a phone from one network on the other due to fundamental differences in hardware.
Plus there’s no guarantee that Verizon will actually be better. VZW says their network is ready, but we’ll have to see what happens after all those people start turning on their iPhones on Feb 10.
Verizon hasn’t announced plan pricing yet, so we’ll have to wait and see how the two networks compare on a monthly basis.
However, your costs will be offset if you (and you should) decide to sell your old device. An iPhone 3GS can go for at least $300 on Craigslist, so that should help dampen the blow.