It's the same for Moggy Mix.
When I got Monty, I thought that I'd feed him on the best I could buy - which at the time was the Science Diet. Then I saw how much a week it was likely to cost. Jeez - he could have fresh fish most days for that!
So then I asked Battersea what they recommend and they said Whiskers, which is what they feed them all at the cattery there. So Monty eats Whiskers, as bought from Wilcos at a reasonable cost. He seems quite happy and is generally a healthy mog.
I reckon that pet food is a swizz. Sure, Iams is probably better quality than Whiskers, which is better than Tesco Value, but when they say it's Rabbit or Tuna or whatever, have you seen the percentage of Rabbit or Tuna that actually goes in there? Unlike human grub, they're under no obligation to tell you the ingredients and from what I hear, they use all sorts of nasties. Including, for some reason, caffeine...