Two readings. zhèng = to earn or to struggle free (挣钱, 挣脱), the everyday one. zhēng is bound, used mainly in 挣扎 (to struggle, to writhe). If it means 'earn,' it is always zhèng.
usage
挣钱 is the colloquial way to say 'earn money.' More formal alternatives are 赚钱 (same meaning) and 收入 (income).
Hand radical. Struggling and earning are both hand actions: 挣扎 uses the hands to break free, 挣钱 gathers with the hands. The radical anchors both senses in physical exertion.
争 supplies both sound and sense for 挣. 争 itself depicts two hands tugging at an object, a perfect echo of 'struggle,' so the whole character is a doubly-handed struggle: one hand from the radical, two more inside 争.
