Can you afford it ?

 

The כף החיים (694:20) writes if one can afford it he should give a silver coin equivalent in weight to an original מחצית השקל.  If no such coin is available it's prefable to at least give the value of that amount of silver. A מחצית השקל weighed 3 Dirhams 1 Dirham = 3.3 grams X 3 = 9.9 grams (silver) Todays cost of a מחצית השקל would be approximately at .85 per gm.x 9.9 =   $8.41 The Minhag is to give 3 Machtzis Hashekels. Only one מחצית השקל need be of that value. The other two can be of minimal value unless one wants to be מחמיר (and can afford it) to give 3x 8.41  =$25.23 As a weight unit, in later medieval and modern Persia the dirham varied between 3.2 and 3.3 grams.