Huh? The Data Pro with Tethering plan is $45 total. It's $30/mo for the grandfathered unlimited, $25/mo for DataPro with 2GB, and $45/mo for DataPro with Tethering with 4GB. Those are your options.
But yes, if you have the grandfathered plan and don't mind jail-breaking, that would be your best route. If you want to stay legit, then it would be $45/mo for a total of 4GB of data to be used between your phone and tethered device(s).
Of course, right now the iPhone on AT&T does not have the Hotspot tethering option, so you'd have to tether via Bluetooth or USB without jail-breaking. We assume that when iOS 4.3 is released, AT&T will enable the Hotspot, but they have not officially said so.