Funnily enough I was thinking about the same thing the other day.
I regularly fly between South Africa and the UK. Usually Air France via Paris or Lufthansa via Munich/Frankfurt. BA and SAA are just too dear. If I book say 4 to 6 weeks in advance the return trip cost me about £500. (which is not bad for a 6000 mile trip!) If I leave it till a few days before I travel it would cost anywhere between £800 and £1200 (in which case I just wouldn’t do it of course)
But I have noticed, particularly in the SA to UK direction, there are around 40% empty seats. Great for me, ‘cos I get to stretch out and sleep on 2 or 3 seats. The flights are same time every day – so what would they do if you rolled up one evening with a small suitcase and said “how much for a seat to heathrow”. They know they’ve got maybe 50 empty seats. Would they do a deal, or would they rather fly half empty? You’d have to be prepared to walk away if they said no and try again another day.
Incidentally I checked flights last night and I can get from UK to SA for £220 and back for £190 fully inc if I fly next week and stay for a month. You just have to be really flexible with your stay time. Silly money!