I'm not 100 percent but I believe your coin is: Constantine I, AE follis, Rome, 316-317 AD. IMP CONSTANTINVS PF AVG, laureate, draped, cuirassed bust right. SOLI INV-I-CTO COMITI, Sol standing left, chlamys across left shoulder, holding globe and raising right hand. Captive at foot left, bare headed, bowing head, hands tied (captive type Helvetica 6b, not in RIC for this issue). Mintmark RS. RIC VII Rome 52 var (captive type), Rare.
Hi. May I ask about the meaning of RS pertaining to the mint mark? Then what value could this rare coin have. Thanks a lot. Charles
R refers to Rome mint, while the S I believe stands for the second workshop (secundo). the rating of rare will not make this coin worth very much, especially in this condition. In fact, I would doubt it being worth any more than $5 since Constantine I coins are so available. If the coin were in pristine condition with the "rare" rating, this coin could be worth maybe as much as 10x this amount. Of course, this is only my opinion. Others may form their own opinion.
I just took a look on acsearch.info and found no coins like yours from the Rome mint, and only one from the Aquileia mint which sold for 75 Euro. It is in better condition, but not by much. Take a look at: http://www.acsearch.info/record.html?id=713245. I will keep looking to see if I can something more like yours.
Here is one from Rome that is selling for $35, but again it is better condition: http://www.vcoins.com/en/stores/cer...soli_invicto_comiti__rome/456250/Default.aspx
There are many, many minor variations of the Sol reverse Constantine coin including the presence or placement of a captive, mint cities and workshop designations. Most collectors who want a coin of the type, will not be particularly anxious to have this exact one or even care whether it is relatively rare but will prefer a coin in better condition. There may be someone out there trying to assemble a complete set of all the minor variations of the Sol type and that person might be willing to pay more if he lacked the exact variation but I'm not even sure there is anyone who collects the type let alone anyone who wants them in this low grade. Ancients are not like moderns. No one is trying to get a complete set of the millions of variations. Bing suggested a value of $5. I believe that was very generous. You might find some interest in a page I wrote several years ago about a similar coin: http://www.forumancientcoins.com/dougsmith/feac27.html