Wow! Look at this coin. It is beautiful, isn't it? I'll bet it is worth a lot of money. Well, not as much as you may think.
This is a modern commemorative coin. Beware of modern commems. You can get stung badly unless you know precisely what you are doing.
Somewhere in the mid to late 1900s, national governments figured out that they could make oodles of money by producing nice-looking coins and selling them to collectors for outlandish sums. The Cook Islands is one such government, but just about *every* government does it today, including the US, Canada, Europe, and everywhere else. Collectors are suckers for nice-looking coins, and non-collectors think if they buy a fancy coin today it will be worth tons of money tomorrow. This is seldom true.
Modern commems are worth their weight in precious metal, nothing more. Even though they come in fancy packaging, and even though they have official Certificates of Authenticity, they are worth only their gold and silver content. If you are buying or selling modern commems, do so within a few percent of the precious metal price. Always find out how much gold or silver a particular coin contains. They are beautiful items, but don't spend too much on them.
For this Cook Island coin, there are 9.6 grams of 90% gold in it. That means there are 9.6 x 0.9 = 8.64 grams of pure gold, the remaining weight is for non-precious metal. Converting the gold weight to troy ounces, 8.64/31.1 = 0.277 ounces of gold in the coin. The current price of gold, according to kitco.com, is $1656 US dollars per troy ounce. Tomorrow the price will be different. Check it daily. A 1976 Cook Islands 100 dollars is worth 0.277 x 1656 = $458 today. I've seen them on sale for north of $1000.