According to wikipedia, Magnentius was the commander of the Herculians and Jovians, the Imperial guard units of the Roman army. When the army grew dissatisfied with the behavior of Emperor Constans, it elevated Magnentius at Autun on January 18, 350. Constans was abandoned by all except a handful of retainers, and he was slain shortly afterwards by a troop of light cavalry near the Pyrenees. As such, Magnentius is a usurper of Rome, not really an emperor.
Magnentius minted many different coins between 350 and 353. The pattern with chi-rho on the reverse is one of the popular ones. The denomination is double centenionalis, made of base metal with weights between 7 and 10 grams and diameters 25 to 27 mm. The obverse inscription reads D N MAGNEN_TIVS P F AVG. Very approximate catalog values are:
worn: $100 US dollars approximate catalog value
average circulated: $400
well preserved: $1000
The coin in our image sold for 170 British pounds, about $220 US dollars, during a 2015 auction by Roma Numismatics in London. CoinQuest thanks Roma for use of their coin image.
It is nothing for modern technology to make accurate replicas of ancient Roman coins. The example on the right is one such counterfeit. If you are buying, never spend any appreciable amount of money with a dealer you do not completely trust.