We are far from experts in ancient Roman coins, but 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. This is an approximate retail price. 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.