It's all very well extolling the virtues of 15kg bags of food for a tenner, but bear this in mind. A 20kg dog on Wagg needs around 400g a day - a 20kg dog on the food I buy needs around 200g a day. That makes your cheap food not quite as good value, in fact the equivalent of £20 a bag compared to the £25 I pay for a natural food which does not contain cheap cereals such a wheat.
Add to that the fact that a lot of that 400g is going to come out the other end because a dog cannot digest it properly, that wheat is more likely to cause intolerance problems, and they probably use chemical antioxidants and preservatives and I think you're beginning to see that there is a difference between cheap and cheerful foods and better quality ones - and it's not just price.