Yeah, definitely Byzantine. Probably from the 6th century AD, though it doesn't look like it's in the sort of condition where it could be identified further, unfortunately.
I agree with Gao. Sixth century follis or 40 nummi. From the pic of the obverse I do not think it will rever be identifiable as to emperor unless someone can extrapolate a unique type from the reverse. Value? A couple dollars as a curiousity. If it were identifiable to emperor and vg condition maybe $8. In fine about $20, and vf and up higher. These coins, while being common in lower grades, are scarce in good VF and higher. I have seen gXF's go well into the hundreds. Chris
