How much will a full grown English mastiff eat?
It really all depends on what you feed the dog.
A mastiff is going to eat a lot, no matter what you feed him, but the better the dog food, the less he is going to need to eat. People look at cheap dog food as less expensive, but that is only true at the register. If you are buying really good dog food, you are spending more, but buying it less, so it really equals out. They will not only eat less, but they will poop less and have healthier skin and coat, which will ultimately lead to less shedding and that's a definitely plus, with a Mastiff.
Anything with corn, wheat and soy is not considered to be real good dog food. Feeding dog food with corn is like giving your dog a bowl of Fruit Loops, twice a day; it's pure sugar. Also, think about the last time you ate corn and how it came back out.
As Matt was saying, there are special needs dogs that do need things like grain free dog food, or something other than chicken, for the main protein source (lamb, beef, venison, fish, etc) and that's where it really starts getting expensive. A dog owner should be prepared to spend a little more to keep the dog healthy. It's worth it, in the end.