These coins were struck to commemorate the royal visit to Guernsey in 1989. They were made in copper-nickel. Some proof issues were struck in silver - see their value at the bottom of this page.
While low in mintage (five thousand), these are very modern coins. The 5000 made were basically all bought by collectors. These collectors have been looking at their coins for 24 years now, and a lot are starting to sell them off. The coins are large and hefty, with a cool ship design, so their values are actually pretty high, considering that they are modern commemoratives.
The catalogs report the following values for the usual copper-nickel coins:
worn: $3 US dollars approximate catalog value
average circulated: $4
well preserved: $6
fully uncirculated: $15
If you have a coin with magnificent eye appeal, with a full, strong strike, and not even the slightest scratch or loss of luster, expect a catalog value of $15 or so. A collector would pay $8 to $10 for such a coin. A dealer would pay less, perhaps $6 or so.
The silver proof issues have the same weight and size as the copper-nickel issues. They catalog at $40 dollars, and contain 0.84 troy ounces of precious metal. It can be difficult to tell the coins apart for a novice collector. An expert silverbug will easily identify the sheen of the silver coins.
If you are unsure, see if your coin comes in a box with a certificate of authenticity. If it does, it is probably a proof issue. If it does not, you can safely assume that it is not a proof issue, and thus struck in copper-nickel.