Generally speaking, it is usually cheaper to upgrade your flight in advance online or over the phone than at the airport before checking in. However, if you wait until you get to the airport, there may be last-minute deals available that could save you money.
How much should you bid for an upgrade to business class?
A good rule of thumb for bidding is to figure out the difference between the cost of your ticket and the cost of an upgraded ticket and bid 20% to 40% of the difference. There's no guarantee you'll win your bid, but if you get lucky it's an upgrade at significant savings.