According to Wikipedia, Constantius II was Roman Emperor from 337 to 361. The second son of Constantine I and Fausta, he ascended to the throne with his brothers Constantine II and Constans upon their father's death.
The beautiful coin in our picture comes from Auktionshaus A. Tkalec AG in Z�rich, Switzerland where it sold in 2009 for 120 Swiss francs, about $130 US dollars. You can get a good idea of value from Tkalec's price. Remember, of course, if your coin is in worse condition that this one, the selling price will be much less. Further, if you determine that the retail selling price will be $X dollars, a coin dealer generally pays one-half $X for coins he or she buys from the general public.
The value of ancient Roman coins is hugely dependent on the eye-appeal, and is reduced drastically if a coin is poorly cleaned such that the metal is brightly orange, poorly struck such that much of the lettering and the mint mark is off-flan, or severely worn such that the inscription and features are difficult to identify.
The camp gate issues of Constantius II are commonly found in lots of uncleaned coins, and thus this is even more true in their case. Coins with poor eye appeal will catalog around $2 US dollars, while average coins will catalog at approximately $15.
There are many minor varieties within this series of coins which will not be apparent to the novice collector. We highly suggest going to your local library and borrowing 'The Roman Imperial Coinage Vol. VIII: The Family of Constantine I ' - this is without discussion one of the most comprehensive books on these coins, and it will list the many bust and mint mark variations along with their respective rarities.
CoinQuest thanks A Tkalec for use of their coin image. It is a beauty!