Best and Cheapest Car Insurance in Illinois. Introduction Car insurance in Illinois can be expensive, depending on the coverage you need and the company you choose. A basic policy for a small car may cost $100 a month, while a comprehensive policy for a large car may cost $200 a month. Definition of car insurance Car insurance is a mandatory requirement in most states in the United States. The amount of insurance a month will vary depending on the state, but is typically around $100. Overview of car insurance in Illinois Car insurance in Illinois can be expensive, but it's important to compare rates and find the best policy for your needs. Factors that Affect Car Insurance Rates in Illinois There are a number of factors that affect car insurance rates in Illinois. Age Age is a major factor when it comes to car insurance rates. Driving Record If you have a clean driving record, car insurance rates in Illinois may be lower than if you have a history of traffic violations. Vehicle Type Location Average Cost of Car Insurance in Illinois.