The first known locally made Roman "money" (in quotes, because the word and concept hadn't yet been invented) was the ponderous lump of bronze called the "aes grave", which started to show up in the 6th and 5th centuries BC. "Aes grave" meant, with typical Roman imagination, "a heavy lump of bronze," and they certainly weren't small change. The lumps were highly irregular, weighed as much as seven pounds, and were traded by weight -- this seven pound lump of bronze for that wagonload of fodder, etc. In 289 a three-man board of "aeris flatores" ("bronze melters") was appointed, and they tried to standardize the weight and purity of the lumps. A fairly well standardized "aes signatum" (a "marked" aes) soon came into circulation. It had a value approximately equal to a Greek silver double-drachma, weighed about six pounds, and was about 7 inches long and four wide. You still needed slaves to carry your purse.
Twenty years later Rome issued its first true coins, the "new" aes grave, a round bronze cast coin now weighing one pound. Smaller denominations went down to 1/12 of an aes. The 1/12 aes coin was called the "uncia", which gave us the English word "ounce" (the twelve-ounce pound is still used in measuring weights of precious metals.)
Meanwhile, Rome also started buying silver "Roman" coins struck at Greek colonial mints in the Campania and Neapolis, now Naples. There is no evidence at all of what Rome may have traded for these silver coins, but it could have been bronze, which, until iron forging became practical, had a fairly high intrinsic value as a raw material for weapons and implements. The silver coins were much handier, and so it wasn't long before silver was the coin of the realm. By the time Rome started striking its own coins, the value of silver had risen and silver coins shrunk correspondingly. Silver coins called "quadrigati" (with a quadriga chariot stamp) and "victoriates" (commemorating victory in the Punic wars) circulated semi-simultaneously for about 50 years.
About 211 BC, the relationship of bronze and silver currencies was "permanently" fixed: a new silver coin, called the denarius, was minted and marked with an "X" meaning that it was worth ten bronze aeses. A half denarius carried a "V" to indicate five aeses, and a quarter denarius, (also called a sestertius), worth 2 1/2 aeses, was marked with an "H" -- the uprights of the H being the Roman numeral II and the crossbar perhaps the 1/2. The weight of the silver denarius was between the quadrigatus and the victoriatis. The smaller victoriatis continued to be minted for circulation in the provinces for another 60 years. From then on, the denarius was the standard coin, but values shifted. Good silver eventually drove bronze out the market, and the weight of the bronze aes slipped below one ounce. Bronze coins stopped being minted altogether for a time, and even the silver sestercius (now at four asses) was too small to effectively circulate -- as in, "what can you buy for a penny?"
Silver coins were quickly debased. Coins were minted of cheap metal with silver plating as early as 89 BC, but that didn't last long. The public demanded good coins, and people could just use a knife to cut into the edge of a coin to see if it was real or plated. In the next reform, coins with pre-serrated edges were issued. Nero debased silver coins again in the mid-60s AD by alloying the silver with cheaper metals, and that was virtually impossible to detect. It set a precedent for later emperors who essentially swindled the people in order to put more coin into circulation.
By one AD, a tiny new bronze aes (now called an "as") had been minted, but it had no real value and was only used as a token to gain entry into the baths or free public performances. Baskets of "asses" stood outside the door, and persons entering would take one and give it to the doorman or ushers, who then used them to keep track of how many people were inside.
Although the official central mint was always at the temple of Juno Moneta (at last, we get to where the words "money" and "mint" came from) on the Capitoline Hill in Rome, eventually more coins were issued in provincial military mints than in the metropolis. Generals thought it wise to pay their armies with good coin. In the late Empire, new coins called the "nummus" (silver) and the "aureus" (gold) were introduced to try to restore confidence after tremendous periods of inflation.
There were hundreds of millions of coins issued in ancient Rome, and many are on the market here and abroad. Most of what you will find for sale are real -- there are so many coins on the market that it's not worth while to counterfeit the common ones. Gold coins, of course, should only be bought at well known specialty houses, and never never never think you will get a bargain there or anywhere else on the open market.
What was it all worth? Roman money circulated for at least 700 years (1000 if you count the early "bronze lump" years), and there were always shifts of value due to changing values in local metals markets and to inflation caused by state policies, including debasement. Talking about the "worth" of "Roman money" is vaguely equivalent to discussing the value of "Western Civilization" money and thinking about everything from Spanish Doubloons to Canadian Quarters. There is some information about buying power at particular places and times, and the most informal records -- those not demanded by central or provincial bureaucracies -- are probably most trustworthy.
People have spent their entire careers trying to assess the value of Roman money, but you don't have to. Instead, you can read some experts' summaries on the Internet at the following sites:
From the Ancient Coin Market site: http://www.ancientcoinmarket.com/ds/worth/1.html
Ancient price information: http://home.sprynet.com/~shpion/anprices.htm
How the poor survived: http://www.hazlitt.org/e-texts/poverty/ch6.html
Encycopedia Britanica on Roman Money: http://www.britannica.com/bcom/eb/article/8/0,5716,108318+1+105949,00.html
Smith Dictionary on what the "salary" of Roman officers and professionals really was -- contrary to myth, Roman soldiers may have been paid "salt money" but were never really paid in salt: http://www.ukans.edu/history/index/europe/ancient_rome/E/Roman/Texts/secondary/SMIGRA*/Salarium.html
A time-line of ancient money is at: http://www.ex.ac.uk/~RDavies/arian/amser/chrono1.html
Info on Roman Coinage is at http://mywebpages.comcast.net/dougsmit/
P.S.: The English words "money" and "mint" came, as indicated above, from the name of the Temple of Juno Moneta (Juno of the Warning). The Romans built that temple on the Capitoline Hill at the site where the famous Roman geese warned the town to the approach of enemies.
Nobody knows how "salary" came to be associated with the myth of paying soldiers in salt. In ancient Roman times there was already some speculation that the Latin word "salarium" might be derived from "sal", meaning salt, but the ancients knew it was just guesswork.
The Latin word for money or wealth was "pecunia" and the ancient Romans guessed that the word was somehow associated with a similar word for what was bartered in earlier transactions, livestock. They knew it was just another semi-educated guess.
The "pound" (as in UK Pound) clearly is associate with the Latin Libra (now Lira) and other currency names around Europe. "Libra" originally meant "scale" or "balance" but soon was associated with the weight of that "new" bronze one-pound "aes grave" of 269 BC. Modern experts speculate that the new coin actually defined the weight of a "pound" -- everything thereafter was weighed out on a balance counterweighted with the new coins.
The word "cash" either came to English through French from a Latin word for "box". Or, alternatively, it has nothing to do with Rome or anywhere else in Europe, and comes from the indian Ocean trade where both Indians and Chinese had coins called "cash." Pure speculation.
The American "buck" did not come from the X on the $10 gold piece, which was called a "sawbuck" because the X looked something like what lumbermen used to hold logs for sawing. "Buck" was in common use in Asian/European trade long before the $10 American coins were issued. Once again, experts can only speculate about its origin, but at least by the 18th century all the languages in the Indian Ocean trade were using variants of the word to describe the one-ounce silver coins (thalers/talers/dollars) that were demanded by Indian Ocean traders. The word "baksheesh" is thus a first cousin of the American "buck."
Finally, "speculate" and "speculation"
most likely come from the Latin word "speculum" which meant "image" or
"mirror" and which is associated with the practice of trying to tell the
future by looking at shimmering reflections in bowls of water.