I have one large-ish (and somewhat overweight despite being on a restricted diet) cat. I don't have his exact food cost breakdown but between food and litter we spend about $12 per month, it's probably about half and half. He eats Science Diet food from the vet. I just looked on the bag and the main ingredient is chicken, although it does have wheat in it too along with a bunch of other ingredients (pork lard, chicken liver... yum).
On the topic of water intake, a vet told me that along with cat fountains and keeping water away from their food, putting out water dishes throughout the house encourages them to drink more. It also helps raise the humidity in your house slightly, which is good for their skin (and yours, but as the vet told me, you can put on lotion and they can't).
I hate to wade into this home-made/raw food argument but I would like to add, according to Consumer Reports testing something like 60% of pork and 97% of chicken sold commercially in the US is contaminated with things like salmonella, MRSA, listeria, E. coli, etc. I'm sure cats in the wild often eat contaminated food as well, but according to the ASPCA feral cats tend to have about a two-year lifespan. Just because that's how things are in the "wild" doesn't automatically mean it can't be harmful to their health. I'm not trying to say that one way is better or worse, just that there are health risks involved either way.