I picked this up at a coin show yesterday for $7 USD. I know it's Constantine The Great, but any description would help. Is it rare?
CONSTANTINE I Obverse: CONSTAN TINVSAVG; diademed head right Reverse: PROVIDEN TIAEAVGG; campgate with 10 rows of stones and two turrets; star above; SMANTΓ (not sure about the last character) in exergue Reference: RIC VII Antioch 78 These coins have a bunch of minor things which specialists separate out and to which various rarity ratings are assigned. I can't discern the last character in exergue on your coin. This coin was struck at the Antioch mint. The exergue breaks down to SM (Sacra Moneta) ANT (Antioch) and the last character which denotes the officina. For this coin, the officina will be A, B, Γ, E, or Z. I think yours is Γ but it's difficult to tell. Roman Imperial Coinage, aka RIC, assigns rarity ratings. Depending on the unknowns detailed above, its RIC rarity is R3 - R4. However, these book rarity ratings usually don't correlate well with real-world availability, and this coin or this type of coin is readily available and in higher grade. For instance, last year I bought a small lot of late Roman bronzes and there were three of these Constantine I RIC 78 Antiochs of officina Γ, theoretically R4 (!), one of officina E (R3), and one of officina Z (R3) [see the first five images in the above link]. So much for book rarity! $7 is a reasonable price for your coin.
I'm not an expert on Late Roman Bronzes, so I'm sure someone can correct me, but I don't see anything particularly rare in your coin. It is indeed Constantine I, aka Constantine the Great. Reverse shows a fortified gate of a military camp, and the mintmark is Antioch. Not bad for $7, but you didn't get a steal either Edit: Or I could have waited another 5 minutes and let TIF post a much more thorough answer. At least I wasn't wrong.
He has seen better days, but its still a cool conversation piece to whip out at a party. Who would have thunk you could get ancient coins for less than the cost of lunch?
