THE CUSTOM
The Ramah (OC 694:11) mentions the custom of Zecher L'Machtzis HaShekel. The custom is on the day before Purim to give three coins to Tzedakah in order to commemorate the giving of the half shekel that was given during the times of the Bais HaMikdash in the month of Adar. The half shekel was donated to the treasury of the Bais HaMikdash once each year. The original half shekel weighed 9.6 grams (some say 8.5 grams). At the current rate as of March 21, 2016, the price of silver is $.51 per gram. The value thus would have been $4.90 per person. In the times of the Bais HaMikdash, only one coin was given. Now we give three coins. We do so because the word "Trumah" appears three times in the Torah section where the original obligation is discussed (Shmos 30:11-16).

THE DENOMINATION OF THE COUNTRY
The minhag is to give 3 half coins of the denomination of that country. Here in America we would give three half dollars. In England it would be three half pound coins, and in Europe it would be three fifty cent Euro coins. Ideally, one should try to avoid giving a whole coin and a fifty cent coin. In other words, ideally, no Susan B. and Kennedy half dollar. It should be noted that the custom of the Vilna Gaon, as cited in the Maaseh Rav (233), was to only give one half coin and not three.

IDEAL METHOD
The Kaf HaChaim (694:2) writes that the ideal method is to use a coin with the word, "half" on it. The American half-dollar has this. The half Euro in Europe does not nor does the 50 shilling coin in England. The Minchas Elazar (Vol. I #30), however, disagrees with this position and states that there is no reuirement of the word "half" to be printed on it. The Piskei Teshuvah (page 574) writes citing the Sefer Midos v'Shiurei Torah (35:1) that ideally we should have the same amount of silver as the original coins that were used in the times of the Beis HaMikdash. To fulfill this, let's get a bit of a background.

The original Kennedy Half Dollar was created as a memorial to the President John F. Kennedy who was assassinated by Lee Harvey Oswald onNovember 22, 1963 in Dallas, Texas. It is still being minted to this day with some differences.

The original 1964 version was minted in 90% silver. The next version minted between 1965-1969 were only 40% silver. The 1970 set can only be found in silver proof sets that were never released into actual circulation. The coins from 1971 and up are not silver at all.

The current silver melt value for the 1964 coin (90% Silver) is $5.71 and fits the ideal criterion it is 11.75 grams well above our 9.6 gram criterion. The current silver melt value for the 1965-1970 coins (40% Silver) is $2.34 or 5 grams - not quite enough.

The second best way would be to use the 1965-1970 coins and make up the difference with other money to bring one up to the $4.90 amount. These coins, at least, are silver. The third best way would be to use the 1971 coins and make up the $4.90 difference.

SCOTCH TAPING THE THREE
The prevalent custom is to scotch tape together 3 half dollar coins and leave them in the Pushkah so that each person can purchase those coins and fulfill the minhag.

If the shul does not have such a scotch taped three-pack, then a possible workaround is that he should give three whole coins and have in mind that half of each should be for commemorating Machtzis haShekel and the extra half of each should be regular Tzedakah.

To read the complete article, see:
Machtzis HaShekel: 1964 Kennedy Half Dollars - The Ideal Method - See more at: http://www.theyeshivaworld.com/news/headlines-breaking-stories/398246/machtzis-hashekel-1964-kennedy-half-dollars---the-ideal-method.html#sthash.cC6bWiaS.dpuf (www.theyeshivaworld.com/news/headlines-breaking-stories/398246/machtzis-hashekel-1964-kennedy-half-dollars---the-ideal-method.html)