Why So Cheap.

Ancient coins are common right now because lots of them have been found by metal detectors. There are also a lot of very expensive coins as well.

Sixthcents: Ever thought of hunting for gold nuggets?
They are the main targets for detectorists in Australia.
Your U.K. colleagues can't do that!

Go to your appropriate Goverment Departmant and get a mineralogical map of the area that you could be interested in, then read lots of books and magazines on the subject.

Hunting for gold nuggets has a strong hobbyist following in Australia.

Gil-galad and ValiantKnight pretty much answered your question. There are a ton of Roman coins available on the market because of people finding some everyday as well as large hoards being discovered with as many as 100k coins or more. Most people don't realize that for coin collecting, Ancients are fairly inexpensive compared to their modern counterparts where there are mint and uncirculated coins in a finite number driving up prices. Don't get me wrong. There are rare or hard to find Ancients as well. Try to put your hands on a portrait coin of Julius Caesar, Marcus Antonius, Caligula or any coin of Hannibalianus. Then you'll see the prices soar. But generally speaking, you can get into this hobby w/o too much investment. And it's a great hobby, especially if you like history and would like to hold a piece of that history in the palm of your hand. Which brings up a huge difference between collecting Moderns vs Ancients. With moderns it is recommended you do not touch the coins. With Ancients, there are no such prohibitions. These coins have laid in the ground for thousands of years. Your touching them will generally not do them any harm. Good luck to you and I hope we've swayed you a little towards collecting these great pieces of history and art.

Regarding coin prices: A dealer I like had an auction that closed yesterday. He sells coins to the highest bidder at one small advance (usually $1) over the second highest bid. That mean some coins will have one really high bid and other more modest bids and will sell for half what the high bidder was willing to pay. It also means that if two guys with fat wallets butt heads on a coin and both bid way over the 'value', one of them will pay through the nose. Two bids for $100 each might result in one sale at $35 and another at $100 (meaning there was a tie bid received later than yours).

On ancients, it is very much a matter what you are willing to pay for a specific coin on a specific day. The next day a lesser coin might sell for twice as much or a better one for half. It is really a hard concept for new collectors but there is no concrete ' Red Book' value for ancients. In this sale I only won two coins after bidding on 20+. That proves that I am a cheapskate compared to some but the big difference between all of us is which coins cause us to be willing to pay big money and which we won't take no matter how cheap they are. Enjoy the hobby and worry more about the coins than the prices.

Picked up a Claudius assarion in G to VG for $15, out of a dealer's junk box a couple of days ago. LIBERTAS AVGVSTVS rev. About 2/3 of the legends readable.