As everyone says, whilst pre airport upgrades are much more of a certainty they're always significantly more expensive!
On a really empty flight I once got a £450 Economy to Upper upgrade, also a £250 Economy to Premium. The first being a very lucky exception, the latter (In my experience) is more common(ish). I've definitely seen and taken advantage of £650 to £800 economy to Upper on West coast US to UK.
As @
craigthemif says, definitely get to the airport early. If I'm hoping for an airport upgrade I'll make a point of being first in the queue when check-in opens which does mean a lot of hanging around!
A couple of years ago I was at the airport and there were a couple of people hanging around check-in (me included), everyone playing the "who's gonna go first" game. I decided not to chance it and stepped into the queueing channel very early (probably at least 1hr before check-in officially opened). I was very quickly followed by the other hangers(and by that I mean they were 1 or 2 steps behind me).
The result, I was able to secure a premium to upper upgrade using miles, the 'very cross' lady behind me wasn't so lucky as I guess there was only one seat left! Based on what I heard she thought that was VERY unfair
