Hello Sandra -- You have an interesting piece from one of the many interesting corners of numismatics (coin collecting). You have a large medal, about the size of a silver dollar, that commemorates a public or special event, a famous person, or a famous place. Your example is from US Army operations on the Mexican border during the time of the Mexican Revolution. Other examples would include world's fairs, town or state anniversaries, statue dedications, and other events or figures in American history. People call these So-Called Dollars and they are a subset of the broader class of numismatics called exonumia.
If you go to eBay and do a search on 'so-called dollars' you will see listings for many different medals that meet this broad definition. Condition, as always, directly affects value. Medals that are worn, stained, cleaned, or scratched will always be worth much less than those in pristine condition. Typical retail prices range from $10 to $100 US dollars, perhaps rising to $200 for a rare specimen. An average price for a medal in good condition is about $50.
Remember that if a medal sells retail to collectors for, say, $50, a dealer would probably pay $15 to $20 for it. Mark-up on medals is more than coins, because there are far fewer medal collectors than coin collectors.
NEVER CLEAN A MEDAL. CLEANING RUINS VALUE.
You can see a superb web site on so-called dollars at (where else?) SoCalledDollar.com. John Raymond is the numismatist who runs the site, and CoinQuest thanks John for use of the image you see here. At the site, you will find many images of fine so-called dollars in John's collection, and you will also learn about the reference material that is essential for any serious collector of these neat pieces.