The Brits pay a good and fair price to the finder in "Treasure Trove" cases. Fourth Century Roman coins are the Ford Pintos of Roman numismatics. You can buy 'em by the hundreds on eBay. The historical value may be greater than the collector's value and the finder will benefit as a result.
Back in he 80' my friends grandfather would buy buckets of them, elctroplate gold onto them, attach them to gaudy, thick gold chains, and then sell them for exorbitant prices to the rapper wannabees.