What was the biggest savings you ever got on the "cash payment no receipt required" discount?

It seems one should get at least 30% off for whatever goods or service, usually service though, they are purchasing. Say a plumber comes by, and he estimates $500.00. You say “How about $300.00 cash, no receipt required?” so he could get himself some “tax-free income”. It’s a good way for both to save on money and get ahead a touch.