This, no doubt, is a valuable medal. These medals are very large (> 60 mm diameter) and come in gold, silver, and bronze. Based on information I've seen in catalogs, mintage of these medals is very low, with only a few hundred pieces produced. This will drive up price for medals that are in good condition. If the medal is worn, stained, cleaned, scratched, or otherwise marred, value will be low.
The sculptor of this medal, Anna Vaughn Hyatt Huntington (1876 - 1973), is famous for public equestrian monuments, including one of Joan of Arc in New York City.
I have been unable to develop a decent pricing model for this medal, because I have been unable to find reliable auction results for its sale. My pure guesses for problem-free medals are:
BRONZE: about $250 retail price
SILVER: about $450
GOLD: about $3000
Remember, these are guesses only, based on no data. If you can send pictures of your medal, I might be able to estimate more precisely. To do this, start and e-mail exchange using the Contact Us link.