Roman coins
Roman coins came in a variety of denominations and were minted in many more denominations than modern day coins.
The gold aureus was the most valuable denomination. While many denominations were used, accounting was done primarily using the sestertius through late in the third century AD. The sestertius is a large bronze coin very popular with collectors although it's not as common as some other Roman coins. The dupondius was worth half of a sestertius.
The most commonly found coin from the second century BC through the first half of the third century AD is the silver denarius. This popular coin is usually between 18 and 20 millimeters across, and weighs between 3 and 4 grams. A denarius was equivalent to four sestertii in the Roman accounting system.
Despite their age Roman coins cab be very reasonable in price unless they are very rare. Rare Roman coins can sell for considerable sums of money. You can find many types of Roman coins on the ebay listings, a great place to start buying Roman.