Both AT&T and Verizon's data plans overlap in the 2GB an 5GB allotments and cost $30/mo and $50/mo respectively. Meanwhile, AT&T offers a lower end 250MB tier at $14.99/mo, while Verizon offers 10GB tier for $80/month. These are month-to-month no-contract plans like on the iPad 2.
Update: Though these plans were taken from Apple's Online Store, AT&T has gotten in touch with us and pointed out that their $30 data plan actually offers 3GB/month of data, making AT&T's $30 plan a better deal than Verizon's.
Update 2: Verizon has also reached out to remind us that the carrier offers a 1 GB plan priced at $20. It seems that Apple's site was offering some outdated or incorrect information on data plans when the store was first coming online. Apple's site is now showing the proper options, although Verizon also offers one additional plan not shown on Apple's site: 10 GB for $80.