However, according to Amtrak's website, as of my knowledge cutoff in September 2021, the one-way fare for a standard fare ticket from Chicago, IL, to Milwaukee, WI, starts from approximately $10 and goes up depending on the date, time, and service type you choose.
Since prices and availability are subject to change, I recommend that you check Amtrak's official website or contact their customer service for the most up-to-date information on fares and availability.