Between 1892 and 1954 the US mint produced a series of gorgeous commemorative half dollars. Collecting a complete set is a lofty collecting challenge but, if you do it, you will have an amazing set of coins made of 90 percent silver with beautiful art work and lasting value.
Harry's example is a Lexington Concord Sesquicentennial, commemorating the 150th anniversary of two famous battles fought in 1775. According to The Grey Sheet, a conservative price guide for US coins, catalog values run like this:
XF: $62 US dollars approximate catalog value
The abbreviations XF, AU, MS60 and MS65 are numismatic grades that quantify the state of preservation of the coin. XF means 'extra fine' and this indicates a well preserved coin, much better than an average circulated coin. The MS abbreviation means 'mint state' or uncirculated. Collectors have gone so far as to assign MS grades from a low of 60 to a high of 70. MS60 coins have never been circulated, but have distracting marks inflicted during the minting process. MS70 coins are perfect in every way.
Since Harry reports that his coin has corrosion, it will be worth less than the XF price.
Grade is not the only factor that affects coin values. There are many other factors. Be sure to read about them on our Terminology page.