Depends on how you measure it I suppose.
Assuming you have all camping gear already and you purchase no trinkets on site...
Basically say that it is £150 per ticket
Transport roughly £25 each (we drive)
Food budget of £5-£10 per day (£7.50 as an average)
Booze budget varies but lets assume £20 per day
For the Wednesday-Monday festival it costs my wife and I about £600.
If you simply take ticket price then you get camping and bands for about £25 a day. Pretty much the same price as many gigs you may go and see!