"DuckDuckGo, which aggregates information obtained from 400 other non-Google sources, including its own modest crawler.)" I looked into it, and it seems DDG is using Bing and Yahoo search API and lots of other sources. I looked into the pricing of Yahoo's search API / Bing search API,it ranges from $0.80 / 1000 queries to several dollars per thousand queries. It seems to expensive to be economically viable with ads,…
They've negotiated a lower rate?
For comparison, that works out to ~$15 per year for the average users search volume.
If you can't make $15 per user per year on search, you have bigger problems...