I second what subject2load has said. If you have already booked you can check under Manage My Booking > Pay to upgrade and see the cost. You will be charged the fare difference based on the fare at the time of upgrade + any change fees, depending on your ticket type (Flex, Flex Plus etc).
The cost at the airport in my understanding is identical to the cost you'll see online or get via the call centre (possibly +/ a couple USD).
EDIT: Also note that you need to upgrade for cash at check-in. If you are Skywards Silver or above you can upgrade with miles even onboard.