In the Attic dialect the plural genitive of draxmh/ had a circumflex accent on the last syllable, as normal for a first-declension noun. The feminine genitive plural of xi/lios also received a circumflex on the last syllable, but (according to LSJ s.v. A.3, citing Herodian) only when xi/liai was used as a feminine substantive meaning "a thousand drachmas" (see web address 1).
From the scholia to Aristophanes, Acharnians 1055, where muri/wn draxmw=n appears (see web address 2).