The 737-400 burns about 35kg/min in the hold, so at 6mins/lap is about 200kgs. The cost depends on the price of fuel but as Static pointed out the total operating costs crank it up to about £500/hold.
A long taxi & holding point delay, say 25mins = about 300kgs in the 737-400.
Prize for the most accurate guess as to how much fuel is burnt LHR taxing and holding every day.