The Isle of Man has discovered that it is very lucrative to issue coinage. They have issued hundreds of different coins over just the past few years.
Manxman (an inhabitant of the Isle of Man) noble coinage is bullion coinage. That is, it is used to invest in precious metal. Most noble coins are made of pure platinum, although gold and silver have also been used with the noble designation. Read the inscription on the coin. You will see words such as '1 oz silver fine', 'gold 1/10 oz fine', or '1 oz platinum fine.' Once you know the precious metal content you can easily calculate the value.
Requester Ray has a 1983 platinum noble. It is made of one ounce of pure platinum. To find its value, look up the value of platinum on a web site such as kitco.com. Today's value is $1333 US dollars per troy ounce. Tomorrow it will be different, so look it up. Ray's coin is worth $1333 today.
When buying or selling bullion coinage, dealers take a margin for themselves to pay for their time and overhead. For small transactions, less than, say, $2000, figure margins between 10 and 20 percent. Larger transactions engender smaller margins.
To add confusion, the IOM has issued nobles with a mixture of precious metals. A 1995 1/4 noble has both gold and platinum in the coin. Since the ratio of gold to platinum is not common knowledge, it is not clear precisely how much this coin is worth.