In my case I'm associating this service more to a news-service (e.g. arstechnica.com, golem.de) => I had no problems paying for their general subscription (3-5$/month) even if I don't ever read all their articles => I would most probably handle a search-engine the same way (and in this specific case limitations like e.g. max 10'000 search queries per month when using a "basic"-subscription would be ok for me).
