The Isle of Man, like many governments today, has figured out that it can make boatloads of money by issuing fancy coins and selling them to collectors. The IOM is especially good at this. Coin catalogs are filled with pages and pages of IOM coins made specifically for collectors. They are all worth their base value, i.e., face value for coins made of non-precious metal, and bullion value for coins made out of precious metal like gold and silver. Often the IOM artificially limits the production of coins to try to drive up prices. It seldom works.
This coin, with Charles and Diana as pictured, comes in several variations:
Copper-nickel: non-precious metal
Silver: 0.841 troy ounces of silver
Gold 5.1 grams: 0.0613 troy ounces of gold
Gold 7.96 grams: 0.2347 troy ounces of gold
Platinum: .5882 troy ounces of platinum
Look up the current values of silver, gold, and platinum to get the value of the coins with precious metal. If your coin is made of precious metal, it will be housed in a package that so states. Copper-nickel coins are sometimes found packaged and sometimes found loose. Unless the package says silver, gold, or platinum, assume you have a copper-nickel coin.
The Isle of Man government sells these coins for much more than they are worth. You can buy them directly from government sources. But, when someone asks 'how much is my coin worth?' they do not want to know what they can buy them for, they want to know what they can sell them for.
You can sell coins made of precious metal for about their precious metal content. For copper-nickel coins you can sell them roughly as follows:
worn: approximately $1 US dollar value
average circulated: $1
well preserved: $2
fully uncirculated: $4
fully uncirculated with exceptional eye appeal: $6