They have had sponsored links for quite some time now, but I wonder how that pairs up with BOSS API not being free anymore.

BOSS queries are $0.80 per thousand queries, so $0.0008 per search. A web search site that uses a premium sponsored link feed should be able to generate about $0.04 per query (50% share of the $0.09 per query that Google makes). Thus the revenue generated could be about 50 times the cost of the raw materials (the BOSS search results). BOSS is cheap in that regard. If you extrapolate to an entire year, DDG could be making about $15 million per year in revenue if they chose to show ads aggressively.

Pardon my ignorance on the matter of ad revenue, but $0.04 per each search query? That sounds very high.

Couldn't they just scrape Yahoo? Yahoo is very lenient towards scrapers - you can survive with just 10 proxies cycling, I doubt they do any intelligent blocking like Google

And risk their entire business on some fragile and easily blocked scraping? (yahoo WOULD notice 1 million scrapes a day)

Yikes

Their entire business is already quite fragile. And I know people who scrape Yahoo close to 1 million queries a day. They really are that ignorant or indifferent.

My thoughts exactly. Maybe I'm wrong but I was under the impression sponsored links were not adequate to fund the growing site.