I'm sorry to say "not much." Unofficial plating of a coin destroys its collector value, and the gold put on is usually so thin that it adds no worth to the piece. If these are silver coins then they have intrinsic precious metal worth, but you have to be careful here--US quarters, for example, are issued in both silver and ordinary clad proof versions for collectors. Ordinarily I'd expect people to gold plate the cheaper coins, and then sell them for huge markups. Television advertisements offering gold plated or gold replica coins are terrible investments, and quite often legitimate coins can be bought directly from the US mint at prices far below what are asked for on TV or in magazine advertisements. Always do research before buying--if the deals were so good they wouldn't be selling them!
Plated coins are considered to be altered items with no extra numismatic value.
Farthings were made from bronze. If you have a silver Farthing, somebody has plated it and it is subsequently of no value as a collectible.
These are not made by any US mint, and have no numismatic value. If the coins are silver and not silver plated, the value would be for the silver in the coins.
Modern commemorative silver dollars are 90% silver and 10% copper and are approx. 27 grams in weight. Silver value changes by the hour but sits at around $27 per ounce right now. Besides that it depends on the specific coin.
New Zealand has never issued a silver Halfpenny. Somebody has possibly silver plated it. Modified coins have no collector value.
Not for general circulation as the metal is far more valuable the the face value of coins being minted, however silver may be added to very special commemorative coins.
A 1976 commemorative Montreal olympic coin weighs 48.6000 grams and is made of .925 silver. If you do a little bit of math, that works out to be 1.4454 troy ounces of silver per coin. The price of silver is constantly changing. Its value at this minute is $13.25 per ounce. In terms of silver value, a 1976 Montreal silver commemorative coin is worth $19.15.
The term Double Eagle is used only for $20 gold coins. This is a 1982 G.W. Commemorative Half Dollar. Unfortunately the gold plating killed the numismatic collectible value of the coin. It is now only valued for the silver it contains.
Modified coins have no collector value. Any value the pendant may have would be as a piece of jewellery. The coin itself might get up to £2 GBP if it was in mint condition and had not been silver plated.
The U.S. Mint has made no commemorative coins of any type for JFK. Many gold plated and counter stamped Kennedy Half's exist and are considered novelty coins that have no collectible value.
The Australian 1954 Florin (Two Shillings)(Royal Visit Commemorative), was minted only as a 50% silver coin. There is no evidence suggesting that the coin was ever produced in gold. If you have a gold coin, it is because somebody had it plated. Modified coins have no value.
The Halfpenny has never been minted in silver. If you have a "silver" Halfpenny, it is possibly because somebody has silver plated it. Modified coins have no collector value. For future reference, please provide a year.