It is some kind of charm made in imitation of an ancient silver coin. The Janus heads were featured on Roman Republican coins, but that is not an authentic ancient piece. It may be old (but likely less than 100 years), and it may be silver, and as such may have some modest value as a jewelry item or novelty piece, but I'm afraid it doesn't have any numismatic value as a coin, because it isn't a coin. Just something made to look like one. If you ask how I know, the style is all wrong for what it purports to be, and it's too circular, and the lettering is off, and a number of other things. Still, it's kind of neat looking, and is a nice little charm or whatever it is.
