No general circulation Australian decimal coin could be considered to be valuable.
Any 1981 coin in absolute pristine mint condition might get $2 to $4, otherwise, they are worth face value.
There is one variation, the 1981 20 cent coin minted at the Royal Canadian Mint Winipeg, shows the platypus with three and a half claws. These may have some collector value.
After the abolition of the Guilder, the new most valuable coin would be the 2 Euro coin
Although the Australian 2 cent coin was withdrawn from circulation in 1994, they are still legal tender and are worth 2 cents. There are two Australian 2 cent coins that are considered to be valuable, the 1967 and 1968 coins, both minted at the Perth Mint. What makes them valuable is the missing designers initials (SD) below the lizard and to the left of the front right claw.
Ram darbar coin
As far as general circulation coins are concerned, easily the most valuable Australian Penny is the 1930 Penny and the most valuable Halfpenny would be the 1923 Halfpenny.
The most valuable coin in the world is the Flowing Hair Dollar. This coin was minted in 1794. This coin last sold in 2013 for just over ten million dollars.
These coins are no longer in circulation, but unless they are part of a Proof or Uncirculated mint set or are individual Proof or Uncirculated coins and in absolute mint condition, they are worth 1 cent. The most valuable Australian general circulation 1 cent coin would be the 1968 coin which, uncirculated and in absolute mint condition might fetch up to $28 AUD. There were 19.93 million minted. A reputable coin dealer will be able to give a more accurate valuation.
Mexico's most valuable coin is the 50 pesos gold coin from any year in the 1920's. The gold coin from 1923, 1924, and 1926 is sold for over $2,000 on eBay.
The most valuable American coin in current production is the half dollar. This half dollar is worth 50 cents when used.
The most valuable coin is a 1933 double eagle gold coin. One was sold for $3,000,000 at a bid
The most valuable coin in decent condition would be the 1933 Double Eagle $20 gold coin, worth millions of dollars. The most valuable bill would be the 1890 Grand Watermelon $1000 bill, also worth millions.
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The most valuable coin to sell at auction is the 1794 silver plugged dollar graded SP66 by PCGS. However,the most valuable coin is the 1849 $20 gold piece . It is unique and has an estimated value of $15,000,000.