当(あ)たりPer~, A~, Apiece, Each

Both express the idea of 'each', but with different nuance and usage. ～ずつ attaches to quantity expressions like numbers or 少し to mean 'each' or 'at a time', e.g. 一つずつください 'Give me one of each.' or ちょっとずつよくなってきた 'It's gotten better a little at a time.' 当たり also attaches to Number + Counter, but means 'per' or 'apiece', e.g. １個当たり５００円です 'They're 500 yen apiece.'

Both phrases derive from 当たる, but are used in very different ways. に当たる means 'corresponds to' and often functions as a more formal, indirect way of saying 'is', e.g. '父親に当たる人' ('a person who is [lit. corresponds to] their father'). 当たり is used after number + counter expressions to mean 'per' or 'apiece', e.g. '１個当たり１００円' ('100 yen apiece').

当たり and それぞれ can both convey a sense of 'each', but their nuances are different. 当たり attaches to numbers to mean 'each' in the sense of 'apiece', e.g. １個当たり１００円 ('100 yen each' / '100 yen apiece'). In contrast, それぞれ means 'each' in the sense of 'to each their own', e.g. 人はそれぞれ個性がある ('People each have unique traits').