To use a small investment or effort to achieve a much larger gain; to catch a big fish with a small bait.
他只花了几百元做推广，就争取到了一笔可观的业务合作，这真是以蚓投鱼，收获颇丰。
He only spent a few hundred yuan on promotion but secured a substantial business partnership; it was truly a case of using a small investment to achieve a significant gain, with fruitful returns.
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