Over the past two decades ancient coin collecting has become a significant part of numismatics (general coin collecting). Enthusiasts enjoy holding coins in their hands that probably passed through the fingers of Roman citizens and ancient Greeks. Pretty cool, I must say.
What happens is that archaeologists and hobbyists find hoards of coins buried in ancient ruins and these coins are retrieved, released from historical studies, and then entered into the numismatic marketplace. Often the coins are in horrific shape, covered with grime and time-baked crusts of dirt and sediment. People buy the uncleaned coins and subsequently clean them using various processes. After the cleaning, numismatists are rewarded with ancient coins made of copper, billon (a mixture of copper and silver), silver, and sometimes even gold, and, if you have not tried this modern form of treasure hunting, a few dollars on eBay will buy you a nice stash of uncleaned coins. It's fun to see what you get!
Many sellers will advertise that gold or silver coins have been found in their batches. While it certainly is possible, most allegedly 'unsearched' batches of uncleaned coins have been sorted through by numerous people before they reach the end-customer on eBay. Copper and bronze coins are the norm to find.
Uncleaned ancients range in price as follows:
UNCLEANED ANCIENT COINS
small copper: less than $1 to $2 US dollars each
large copper: $2 to $5
small silver: $2 to $4
large silver: $5 to $15
'Small' is less that about 3 grams in weight. 'Large' is more than 3 grams.
Once cleaning is complete, the next step is attribution. This is where numismatic skill and expertise enter the picture. Sometimes it is enough to see the devices (pictures) on a coin. An owl almost always means the coin is from ancient Athens, Greece. A large SC monogram means the coin was authorized by the Roman Senate. But, to be fully attributed, inscriptions usually supply the most information. Ideally, you would like to read the name of the ancient emperor, caesar, or king. Couple this with the weight and composition, and you know what you've got, and the time period when it was minted.
Fully attributed, nice looking ancient coins can sell for hundreds or thousands of dollars, but sometimes it is impossible to figure out that attribution precisely. Such unattributed coins sell as follows:
CLEANED BUT UNATTRIBUTED ANCIENT COINS
small copper: $2 to $5
large copper: $3 to $10
small silver: $8 to $20
large silver: $10 to $50
There are numerous websites available for attributing your coins. Wildwinds.com is one of the best and most popular websites, with search tools and thousands of pictures of Roman, Greek, Celtic, Bactrian, Seleucid, Parthian and Indo-Scythian coins.