The largest expense is housing. Many larger cities have public transportation. Every city is not the same as it costs MUCH more to live in New York than it does Houston, Tx. For instance, 1K a month MAY get you a one room apartment in New York (someone correct me if I'm wrong). In Houston, 1K will get you three bedrooms, a fireplace, 2 bathrooms and 15min or less away from the medical center or downtown.
The average family income is about 50K a year. Many people are making it individually at under 25-30K a year and attempting to support a family. Their idea of "good" may be different than your's. If your question of what is "good" means what is "comfortable", anyone making over 50K a year can probably make it in most cities living fairly comfortably unless they have 2 or more kids and other expenses.