'cost-saver' originates from Modern English as a compound of 'cost' + 'saver'; 'cost' comes ultimately from Old French and Latin roots relating to price/expense, and 'saver' is formed from the verb 'save' (from Old French 'sauver' < Latin 'salvare').

'cost' was used in Middle English (from Old French forms such as 'coust'/'coste') to mean 'expense', while 'save' came into English via Old French 'sauver' from Latin 'salvare'; the agentive form 'saver' (one or something that saves) and compounding with 'cost' produced the modern compound 'cost-saver'.

Individually, 'cost' originally referred to price/expense and 'save' to preserving or keeping safe; when compounded, they came to mean 'that which preserves money' or 'that which reduces expense', the current usage.