I have heard of this 'bargain cash upgrade' at the airport but never once seen it. Actually that is not true - I did see it at HK airport from HKG-SYD last year.

I have asked a few times at check-in and been told that yes you can pay up to the next cabin but you pay from the ticket you bought up to the no advance purchase price of the cabin you want to move into.

Unless you are extremely fortunate don't expect to see any deal worth considering.