Verse Ex30-13c discussing the obligation of a half-dollar tax for the temple, states, This they shall give, every one that passeth among them that are numbered, half a shekel in holy shekels--the shekel is twenty gerahs-- half a shekel for an offering to HaShem. The underlined phrase holy shekels cross references the currency used for donations made to the Holy Temple defined in Lv27-02:03 and Lv27-25: Speak unto the children of Israel, and say unto them: When a man shall clearly utter a vow of persons unto HaShem, according to thy valuation, then thy valuation shall be for the male from twenty years old even unto sixty years old, even thy valuation shall be fifty shekels of silver, in the holy shekel .... And all thy valuations shall be according to the holy shekel; twenty gerahs shall be the shekel. Hence the Rashi comment:Holy shekel refers to the currency used in Holy Temple matters.

Advanced Rashi: The JPS English translation that we use reflects this Rashi thru a punchy translation. JPS translates holy shekels as Temple shekels: This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary-- the shekel is twenty gerahs--half a shekel for an offering to HaShem. The use of punchy translations to reflect Rashi comments was first advocated in my article Peshat and Derash, which may be found on the world wide web at http://www.Rashiyomi.com/rashi.pdf.