The US issued several silver half dollars as commemorative coins in the early to mid 1900s. They are all beautiful works of art and collectors enjoy building sets of them. Nate's coin commemorates the 100th anniversary of the Monroe Doctrine and shows John Quincy Adams and James Monroe of the front, and a map of the Western Hemisphere on the back.
If you remember your US history, the Monroe Doctrine, as of 1823, made further colonization of the Western Hemisphere by European nations an act of aggression against the United States, while also promising that the United States would not interfere with existing colonies or meddle in the internal concerns of European countries.
The Red Book, a popular catalog of US coins, indicates the following catalog values for undamaged (no scratches, stains, cleanings, etc.) coins:
worn: $20 US dollars approximate catalog value
average circulated: $25
well preserved: $30
fully uncirculated: $100
Convert these to actual buy and sell values using our Important Terminology page.