Microsoft has announced a free tier of 5,000 searches per month as part of its move to monetize the Bing search API. Microsoft told us that the new tier was "thanks to the feedback and interest from the developer community." The pricing structure should come as a relief to many sites, which were understandably upset when Bing first announced it would charge for the API.
Since its launch, the API has been entirely free to use, whereas Google charges sites that use more than 100 searches per day. After a site uses its free quota, pricing starts at $14 or $20 per month for 10,000 data or web searches. Google charges $5 per 1000 queries over the 100 per day limit. The two services deal with their free quotas differently, so it's difficult to do a direct comparison, but it looks as though Bing is cheaper in nearly every circumstance.