Has anyone ever paid "face value" for an ancient? : - ) As for the OP coin, soak it in water for a few years (start with a week or so) and brush it with a stiff brush to see if you can bring out any detail. Good Luck
Not spending all day online usually means I see threads after they have been swiped so I feel odd returning to the topic but everyone knows I'm no fun so: When you can not decide between two ID's, the correct answer is more often the more common. When you have looked at a lot of coins over a lot of years a gut feeling often guides your decision in a way that can not be proven or even justified. I believe this is Constantius II with the last of his reverses showing emperor standing (95% probability). If I had to pick a mint, I'd say Cyzicus (30% probability). If I had to assign a cash value it would be under $1. To be worth something, a coin needs to be identifiable without resorting to psychic activity. The same coin in legible shape might be $15 and $25 if pretty. http://www.forumancientcoins.com/dougsmith/dd04.html
Doug, be careful. There's a US cent trying to sneak into your collection. If you let that happen, pretty soon everything is going to be about slabs and hairlines and greysheet values, and you'll be even less fun than you are now.
That cent is uncollectable, it's not in a slab and was once touched by hands that didn't even have gloves on. Perhaps it is a rare variety that can easily be found with an electron microscope though.