The total value of farmsand rural real estate for sale in Illinois's Warren County recently on Land.com was around $6 million. This represents almost 1,000 acres of farms and rural land for sale in Warren County. Warren County ranks 54th in the state for the combined amount of land currently advertised for sale. Monmouth had the most land for sale. Manufacturing and related services are the largest economic sectors in Warren County. Warren County is Illinois's 49th biggest county (543 square miles). It's located in the Western Illinois region of the state.