As far as investments, I think if you have a solid offering and can get some traction it shouldn't matter. Twitter bought a search product which was completely based on their API plus the technology that the search guys had developed (sorry, I forget their name). The key is what you are bringing to the table. If you are just a site accessing API's, it is going to be difficult. You should have a solid value add.

As far as the 3rd party pulling their API, I think that is a risk you take, but if you are talking about Twitter, where a huge portion of their success is a result of the API, then I wouldn't worry as much. In the case of BOSS, Yahoo! could pull it, but you could probably source search results from elsewhere.

As far as offering these services in a paid offering, unless you are adding something significant which the user is paying for, I would think your success would be challenged in the pay model. People don't really pay for a new view of what they can already get for free (I don't think).