This tethering should be free. Not long ago you could say that very little resources could be used on a mobile phone’s browser and there were 5 dollar data plans for these phones. Akin to trying to drink a pool with a straw.
Today’s phones have much bigger straws now. You can conceivably use the same amount of data through a tethering just like you would in normal use of an iPhone. So for Apple to charge anything is basically double dipping the customer for cash. I would not be surprised if they charged in the range of 10-20 dollars for this feature. Another example of AT&T exploiting it’s iPhone monopoly to abuse prices.