Fortunately for CoinQuest, there's CoinTalk. That's where the *real experts* live. If you click on this link you will see an interesting CoinTalk thread on Conder Tokens and, sure enough, your Dundee farthing is right there.
Conder Tokens, also known as 18th Century Provincial Tokens, were first minted In 1787 in response to British coin shortages in outlying areas. Conder Tokens are named after James Conder who was an early collector and cataloged these interesting coins. They are copper coins (usually pennies and half pennies) minted by towns, businesses, and organizations of all types to meet a need for low denomination coinage that was not being provided by government. Thousands of varieties of tokens were minted, many are beautiful and intricate works of art.
There is a great site on Conder Tokens at (where else?) ConderTokens.net. From there you can visit many other web sites dedicated to these wonderful and interesting collectibles.
As to value, the supply is surprisingly large and the demand is predictably small. This leads to relatively low prices for something so old, interesting, and beautiful. Looking around the web at typical prices, including those on eBay and the sites mentioned above, approximate catalog values run like this:
worn: $25 US dollars approximate catalog value
average circulated: $50
well preserved: $200
Marcy, why not consider starting a collection of these fascinating coins? Don't sell the one you've got. Go out and get some more!
CoinQuest thanks CoinTalk for the use of their image.