Normal usb hubs cannot supply enough power to individual ports needed to
charge an iPad. iPad carts are equipped with high power usb hubs. You don't
need to be connected to the computer to charge. They have their own power
supplies. You can buy just the high power hub which is pretty expensive on its
own, which explains part of the high cost of an iPad cart.
I would look into something like this charging station from Griffin. It will charge
10 iPads in a nice stand and can be expanded to charge up to 30 iPads.
I'm not aware of a cheap way to charge multiple iPads beyond getting a power
strip and plugging in the power adapters that came with the iPads.