The mean cost per item decreased every year between 2003-04 and 2014-15, but has remained relatively stable since, with some year-to-year variations.
Gostyngodd y gost cymedrig fesul eitem bob blwyddyn rhwng 2003-04 a 2014-15, ond mae wedi aros yn gymharol sefydlog ers hynny, gydag amrywiadau o flwyddyn i flwyddyn.