They should charge whatever makes them the most money per flight. Unless a competitor to GoGo comes along, or using cellular devices is allowed on planes, it is likely a good decision short term and long term.
jonafato mentioned in another comment that they could use the price to control usage rates, increasing the price to keep the number of users down and the bandwidth:user ratio up.
jonafato mentioned in another comment that they could use the price to control usage rates, increasing the price to keep the number of users down and the bandwidth:user ratio up.