The United States issued a wonderful assortment of special commemorative half dollars starting in 1892 and running through 1954. They started again in 1982, but the modern commemoratives don't hold a candle to the older ones because the modern designs are full of pet projects and political correctness (that's my opinion of course, not authenticated fact!)

Your 1918 Illinois Centennial half dollar carries catalog values as follows:

well preserved: $100 US dollars
fully uncirculated: $130

These values come from the Grey Sheet, a respected price guide for US coins. Apply the principles found on our Important Terminology page (link at upper left) to convert these catalog values to actual buy and sell values. The darker patina of Alicia's coin may make it a little lower in value than the figures quoted above.