Apple has updated its online store with a ship date for the 3G version of the iPad, which can access the Internet using Wi-Fi or AT&T’s network.

The 3G iPad will begin shipping on May 7 and will be significantly more expensive than the Wi-Fi-only version. The 16 GB version of the 3G device starts at $629 and goes up to $829 for a 64 GB version, compared with the version with just Wi-Fi, which starts at $499 for the 16 GB model and goes up to $699 for 64 GB.

Yankee Group analyst Andy Castonguay says the additional $130 for a 3G version covers equipment costs for the cellular connection.

“From a component standpoint, the extra $130 is probably relatively fair given the additional module that needs to be put in for 3G and GSM/EDGE,” he says. “The majority will probably tend toward the lower-priced iPad, but certainly there are those who need more ubiquitous connectivity.”

Those who purchase the 3G iPad will be able to prepay for monthly access to AT&T’s network. A 250 MB per month plan costs $14.99 or users can get unlimited data for $29.99 per month.

Apple says it sold more than 500,000 iPads during the first week the device went on sale.