I'm puzzled by why you ares eeking such a precise answer.
Are you saying that a few pounds either way could make the difference between going or not going?
Remember, the petrol cost is only part of the cost of driving. The typical marginal cost per mile of a journey is between 30p and 50p-. In addition to fuel costs there is wear and tear on tyres and the engine (meaning you'll need a service earlier), oil etc. And every mile on the clock reduces the resale value by at least 5p a mile. And every mile increases the chance of losing your no claims bonus.
For a round figure I would say 440 miles @ 30p =�132. So go for something between �100 and �150.