This is a coin from the ancient Roman Empire. A quick search at our favorite web site for such material, MA Shops, shows hundreds of these Severus Alexander silver denarii for sale in the range of $30 to $200 US dollars, depending on specific type and condition. So these are an approximate retail price. Catalog values are inflated over retail:
worn: $35 US dollars approximate catalog value
average circulated: $100
well preserved: $275
If you were to sell your coin to a reputable dealer in ancients, he or she would usually pay about one-half retail to buy the coin from you.