It's a bit difficult to see the details clearly on your pic, but I'm guessing Constantius Gallus, and it looks like it could be an Antioch mintmark. http://www.wildwinds.com/coins/ric/constantius_gallus/t.html Read more about Constantius Gallus here: http://en.wikipedia.org/wiki/Constantius_Gallus
It's definitely Roman, and it dates from somewhere between 348-361AD. Can you tell me if the figure on the front is wearing a diadem (basically a headband)? It's hard to tell. Also see how much of the obverse legend you can make out in person. I can make out "..NTIVS" on the front of the coin, which means it's Contantius II if there's a diadem and Constantius Gallus if there isn't (which would date it to 352-354). As for the reverse, it's a Roman soldier stabbing a falling barbarian horseman, and the legend around says "FEL TEMP REPARATIO," which can be very roughly translated to "Happy days are here again," because when the Romans thought of happy days, they thought of a guy getting stabbed. The mintmark in the exergue is a bit hard to make out (let me know if you can read it at all better in person), but I think it begins with "AN," which would mean Antioch.
If it's Constantius II, it's practically worthless in this condition. Maybe $5 at the most. If it's Gallus, it be up to $10, but probably not more.
My first inclination was to read the mintmark as CONSgamma making the coin Constantinople rather than Antioch. The C is weak at the left. I believe the head is clearly bare making the coin certainly Gallus but I do not see a coin in this condition making any significant difference in value between the two rulers. Good appearance means more between a common coin and a very common one. To be worth even $5, a coin of this type must either be clearly a rare mintmark variety, clearly identifiable or pretty to look at. This is not. Uncleaned Roman coins sometimes sell for more unidentified because some people hold out the hope that the coin might turn out to be a rare one. After a coin is IDed as a common type or, worse, it is established that the coin lacks the detail needed to ID it, the cash value drops a lot. If this can be wiggled around enough to read that mint mark with certainty, the coin has some potential for market but still probably under $5.
Looking at it again, you are quite right. Surprised I didn't see that earlier. Edit: And MrsMintage, I'd probably listen to Doug over me about the pricing. He knows much more about this stuff than I do.
