We've said there will be ways to get BOSS without having to pay a fee. This will probably involve including advertising. The exactly details of this aren't available right now though.
Disclaimer: I work for Yahoo! I also have no insight into the Bing API roadmap.
To quote from the Bing Terms of service:
3. CODE OF CONDUCT / RESTRICTIONS ON USE.
You will not, and will not permit your users or other third parties to:
(o) commercialize (i.e., sell, rent, or lease) Bing results;
12. PRE-RELEASE. The API (and related services) are pre-release versions. They may not work the way a final version of the API (and such related services) would. We may change the API (or these services) for a final, commercial version. We also may not release a commercial version, in our sole discretion.
As I read these conditions, so far the Bing API seems similar to how the BOSS API was before this post. Bing may results not be "commercialized". We've announced that ours is going commercial BOSS, that seems like a good thing for actual business rather than just hacking.
Is there any free web search API?