I had the situation once when I was asked to travel - with extreme urgence - from London to Budapest. This meant going to the airport directly, with no means to pre-book the ticket.

I did so, went to T1, and asked BA how much and when. They gave me the "it'll be quite expensive, you know" speech, but in the end, could only offer a £750 flight, arriving 9 hours later. Not great.

So I wandered over to T2, and unwittingly started a bidding war between about 5 or 6 of the neatly aligned counters there. I ended up paying £140 for a return, on OS, leaving in 20 minutes, with a 30-minute connection in VIE (which I easily made, thanks to the 'direct transfer' people). Not bad going!