a sprat to catch a mackerel

A small amount of effort or money to expend or risk that has the potential to yield much greater benefits or rewards. Often used after the verb "set," meaning to use as bait; a sprat is a much smaller fish than a mackerel, so you use the smaller as bait to catch the larger. Primarily heard in UK. When big retailers like them offer cash-back prizes, they are setting sprat to catch a mackerel—the money they make from sucking in customers more than outweighs what they spend in actual payouts.

If you describe something you do as a sprat to catch a mackerel, you mean that it involves a small cost or amount of effort but it will bring you great rewards or benefits. Setting a sprat to catch a mackerel, the store is offering in its summer sale a chrome blender for £99 (was £149) to those who spend £50 or more. Note: A sprat is a type of small fish and a mackerel is a larger fish. Both fish may be eaten.