Apple Ipad 3G Monthly Plan?

So i'm going to buy the apple Ipad 32 GB, and its the 3G, and i'm confused on the monthly plan. So with my AT&T plan i pay for internet on my smart phone (samsung xperia), Like 20 or 30 a month, so would i have to pay extra for my ipad? Or would my monthly 30 dollars a month for my internet phone charge carry over if i set the ipad up on that same plan? Or would i have to pay additional for the ipad 3G service?