No, it's not Rockford. You can blame the high real estate taxes for that. However, the cheapest city to reside in the United States is in Illinois

The Winner: Danville, Illinois! This from the U.S. Bureau of Economic Analysis.

Wikipedia

The City Of Danville website states:

Danville, Illinois offers small town charm and big city amenities. Our city is known for its range of shopping, from antiques and boutiques to big-box, excellent parks and recreation, academics, affordable housing, and quality healthcare for veterans and everyone else. We invite you to discover all that Danville has to offer!

 

 

The Chicago tribune reports that two other Illinois metro areas also ranked among the top 10 least-expensive spots. The area encompassing Cape Girardeau, Mo. and parts of Illinois across the Mississippi River ranked 7th, while Carbondale-Marion area was 10th.

Rockford, IL ranked at #150 out of 381 cities in the country

On the high end, the two most expensive places to live in the U.S. are:

1. Honolulu, Hawaii
2. New York City, NY

The basis for rankings compares the cost of living in a particular area to the national average. Read more here.