Does any airline offer upgrades in exchange for cash at airport check in counters

Have recently heard BA offered to upgrades at check in for £200 from business to first on a flight from LHR - JFK

Is this common, is it worth offering an amount at check in, am travelling business class tommorrow from LHR-BKK with TG confirmed in Business Class. What is a sensible amount to offer or will they just quote the published fare difference between business and first

am intuiged so any help appreciated and of course will be posting full trip report on my return!