£4-£5 seems to be what's charged in a decent pub for a decent beer - but you tend to get a good selection. Wouldn't expect it to be less for event catering. Been to some concert venues where it's been £6 a pint.
That said, if it's about creating an experience, then there has to be something 'different'.
As others have suggested, a couple of tents in the fan zone doing real ale or lagers from a different country each week. Fully justifiable charging a fiver for that.
The fosters - as sole draught supplier should be done on various offers, for instance:
Happy hour between 1:30 - 2:30.
23 half price fosters vouchers with every ST....or half price hot drinks if you don't drink. People more likely to spend on food then.
10 half price fosters tickets with each 1875 membership etc.