As best we can tell, Lawrence, you have an ancient silver Denarius (about 3.5 grams) from the reign of Caesar Vespasian, emperor from 69 to 79AD. The full inscriptions read:
VESPASIANVS T CAESAR IMP with COS on the eagle side.
The example in our picture comes from Gemini Auctions where it sold for $450 US dollars in a 2013 auction. Gemini's coin is in beautiful condition and that's why it commands such high value. Coins with more wear, less eye appeal, or significant problems (gouges, scratches, discolorations, mutilations, etc.) will be worth far less.
Approximate catalog values:
worn: $100 US dollars approximate catalog value
average circulated: $300
well preserved: $600
These ancient coins always stand on their own merits, so if you have one that looks nice be sure to take it to a knowledgeable collector or professional coin dealer for an in-person appraisal.