It depends on the idea, lead quality and market. Organic search to conversion of 3-4% is awesome, super targeted leads maybe 10%. Of course numbers generally are higher for things like newsletter sign ups etc vs handing over cash.

I'll admit I haven't read your link but if you were doing business validation and 1 in 5 people were saying yes and the market is large enough then those are good numbers. If your market is super small in the world then I'd say no. Of course there are a few exceptions to that rule.