There is an account in the Bible (Mark 12 and Luke 21) where Jesus sees a poor widow put two 'mites' into the offering. He uses the occasion to teach that it is not the amount you give, but why you give, that's important to God.
The widow's two mites are actually called leptons. These were the smallest denomination coin of the day in Judea and were struck during the reign of Alexander Jannaeus. Thousands and thousands were made, and you can still find them today. They are very crude coins, and most specimens only have a small amount of detail visible - just enough to identify them. Some amazing specimens have fully legible inscriptions and are well centered. There is a huge difference in value from the low to the high end:
worn: $6 US dollars approximate catalog value
average circulated: $30
well preserved: $350
One of our favorite places for ancient coins is VCoins.com. At this site you will find pictures and prices for genuine leptons.
Thomas' coin, however, is not a genuine lepton. It is a modern reproduction used for promotion. It carries no value.