Microsoft said the charges will start at approximately $40 (USD) per month for up to 20,000 queries each month. The pricing will likely go into affect in the next 3 to 6 months.
In the next several weeks, Microsoft will document the schedules for this transition and during that transition, Microsoft will encourage users to use the Bing Search API for free. Then at some point, Microsoft will start billing you for use.
If you are already using the Bing Search API, keep watch of the developer blog for updates. Those doing 3 to 4 million queries will likely hear directly from Microsoft.
SEOs and Webmasters are shocked Microsoft is going this route. They felt any upside people have to use Bing, keep it, especially for developers and early adopters.
Forum discussion at WebmasterWorld.