Might be Trajan. Trajan 98-117 AD AE sestertius Obv "IMP CAES NERVAE TRAIANO AVG GER DAC PM TRP COS V PP" Laureate bust left Rev "SPQR OPTIMO PRINCIPI SC" Emperor on horseback right spearing Dacian in front of horse
We're going to at least need to see the other side and preferably the diameter in millimeters and the weight of the coin in order to properly identify it. It's definitely Roman, but I'm not comfortable saying much more without the above.
Comepare with similar ones on other sites, like www.vcoins.com. looks like its maybe around $50-100. edit: this isnt the most accurates way though
Thats a nice coin, the denomination could be a Sestertius depending on weight. Quite an interesting thing for your dad to have found, if only the coin could tell us how it came to be where he found it. That is one of my favorite reverses for Trajan. The emperor on horseback spearing a dacian. SPQR OPTIMO PRINCIPI - I always like the coins with the phrase 'SPQR' on them, which many coins of Trajan have. Senatus Populus Que Romanus, the Senate and the Roman People.