As far as I know, DDG isn't paying for access. But I might be wrong about that. Maybe they are they paying for Bing, but not Yahoo?
http://developer.yahoo.net/blog/archives/2010/08/api_updates...
"We are exploring a potential fee-based structure as well as ad-revenue models that will enable BOSS developers to monetize their offerings. When we roll out these changes, BOSS will no longer be a free service to developers."