If you are trying to get the input box values, you will have to find the control like:
GridViewRow row = GridView1.Rows[e.RowIndex];
string price = ((TextBox)(row.Cells[3].Controls[0])).Text;
You can also check against the e.NewValues.
And fyi: You will have to set the e.Cancel=true, if you want to cancel the event upon meeting certain condition(s).
Check RowUpdatingEvent ref.