That was me I only said it as a piss take really, but all credit to Ged, before I even had the chance to wind him up he whipped his wallet out and gave me the twenty quid
And the round of drinks cost me £18
Personally I always try and agree a fee with the promoter before the event. I then expect to be offered that fee, if the venue has a poor attendance it's then up to me whether I accept it. By the same token, if the place is rammed, I don't go and ask the promoter for more money.
If you are a promoter, DJs fee's are a cost of promoting, same as venue hire, flyers / promotion, equipment, and all the thousand other things involved. You should agree a fee before the event, and have that money in your pocket when you arrive at the venue. It's the promoters job to promote the event, if he does it well, people will turn up and he will recoup his costs, if he fails to promote the event properly, people won't turn up, and the loss is his alone, and the DJ, who persumably has done his job, ie played records, should not be expected to shoulder any of that loss.