Originally Answered: What's a good cat food that is not too expensive?
Some of the cheapest high quality cat foods you'll find would be Felidae and Chicken Soup for the Cat Lover's Soul.
Not all pet food is made equally. A lot of it is full of corn, by-products, dyes, unhealthy preservatives, filler grains and all sorts of nasty stuff. A lot of pet food companies are perfectly happy to the dump cheap leftovers and things that aren't safe for human consumption (from human food processing plants) into their foods. Will it kill your cat? No, it has to be nutritionally complete and safe to even be marketed. Is it healthy? Not by a long shot.
Corn is a low quality ingredient you never want to see in your pet food. Corn and low quality grains are two of the biggest culprits when it comes to food allergies in our pets.
Thankfully, there are some excellent cat foods being made these days that include organic, human grade ingredients rather than trash not fit for human consumption.
Examples of low quality foods to avoid: Anything you can find in a grocery store will be low end, Purina, Iams, Eukanuba, Science Diet, Royal Canin, Whiskas, Fancy Feast, Friskies, Meow Mix.
Examples of high quality foods to look for: Innova, Wellness, Solid Gold, Felidae, Fromm Four Star, Merrick, GO Natural, Nature's Variety Prairie, Nature's Logic, Artemis Fresh Mix, Timber Wolf Organics.
Although the high quality foods are more expensive, you're getting what you're paying for. Less filler material means more concentrated nutrients... this means you typically need to feed far less of the high quality food than you would of the low quality one. Which also means less poop!
Before following your vet's food recommendation, keep in mind that vets get /very/ little nutritional training during their schooling. Besides that, what training they /do/ get is usually sponsored or taught by the crappy pet food companies! They also often get paid to sell some of their products at their clinics (Science Diet, Royal Canin etc.)
A great option is to go with an entirely grainless diet. Diets high in grain have been attributed to problems with diabetes in cats. Cats are obligate carnivores, so why should there be grain in their diet? Many of the high quality foods now put out grainless formulas. Some good grainless diets include: Innova EVO, Wellness CORE, Blue Wilderness, Nature's Variety Instinct, Orijen, Horizon Legacy, Merrick Before Grain, Fromm Surf & Turf, Now!, and Sold Gold Indigo Moon, Taste of the Wild.
Some pretty decent foods can even be found in common pet stores. Petsmart carries Blue Buffalo products (such as the excellent grain free diet Blue Wilderness). Petco carries Wellness, Solid Gold, Natural Balance, Eagle Pack Holistic, Blue Buffalo, Castor & Pollux Organix, Pinnacle, and Halo. If you can't find a food, most of the high quality food brands have websites with store locators on them.
Another option, if you can't find anywhere around you that sells good foods, is to order your pet food online. Here's an excellent place to do so: http://www.petfooddirect.com/store/
Remember that foods should be switched gradually (mixing new slowly in with the old over about a two week period), especially when switching to a higher quality one, so as not to upset tummies.
Another option for feeding cats is to feed raw. This is something that should be thoroughly researched before being attempted:
Now the question is, do you feed wet or dry? Wet is the correct answer. The reason is, in the wild, cats normally get most of their water content directly from their prey items and drink very little. Domestic cats are no different, and because of the fact that they are designed to take in water with their meal, they have a very low thirst drive. Cats often just don't drink enough. This leads to urinary tract infections and crystals. The bit about dry food being better for teeth is a myth and has not been proven in the least (cats barely even chew their dry food and, really, does a pretzel clean /your/ teeth? Cats should have their teeth brushed with cat toothbrushes and cat toothpaste at least a few times a week as well as see the vet for dental cleanings when necessary /regardless/ of what they are being fed). Canned/wet food is better because it more closely mimics the cat's natural diet. More on why canned food is best:
http://www.catinfo.org/ (Excellent cat nutrition information by a vet)
Another option to get cats to drink more would be a cat fountain. Cats tend to like to drink from running water and cat fountains see to that need, encouraging cats to take in more water.
Also remember that freefeeding (leaving food down) is the fast lane to feline obesity. Make sure to have scheduled feeding times based on the feeding instructions on your cat's food.
http://www.dogfoodanalysis.com/dog_food_reviews/ (Dog food reviews. It's for dogs, but most of the high quality brands also put out excellent cat foods. Four stars is a decent food, five stars is a great food, and six stars is an excellent food.)