Presuming that you're talking about DS licence as opposed to stewarding or SG before and after public times, then I've heard everything from £7.50 to £10, but around £9 seems normal.
However, you also need to take into account if free or subsidised food is provided because it makes a difference if you're doing 12+ hour shifts and living on site. Although at most festivals it is possible to take your own food if you don't mind being a bit restricted in what you eat.
A lot of festivals do go for the cheapest bidder which tends to push wages down an then you have companies subcontracting to subcontractors, with 3 to 4 levels of company before you get to the one thats actually getting paid by the festival, each of which is taking a cut.