Usually services and APIs have usage threshold, so as to enable experimentation and small-scale use without any hurdles and up-front obstacles, but open up possibility of offering better SLA and additional options with paid usage for apps that need that level of support.
That said looking on programmableweb.com for search APIs is probably an interesting option - see http://www.programmableweb.com/apis/directory/1?apicat=Search.
I am also curious, what you're specifically looking for in terms of capabilities, what you'd like to see in the Bing API etc. Any feedback and I can relay to the team (since I am on the Bing team).
