Yahoo’s Build Your Own Search Service, or BOSS, is one of the most visible components of Yahoo’s Open Strategy.

With BOSS, developers are given the ability to build their own search engines that query Yahoo using an API and can then take the results that Yahoo returns and do with them what they please.

Yahoo’s Open Strategy is designed toopen up the assets of Yahoo in a way never done before” and BOSS appeared to represent a real push to make Yahoo search relevant again by putting it in the hands of third party developers.

Obviously, Yahoo’s new strategy wasn’t exactly charity. With BOSS, Yahoo initially planned to use the offering to “expand the footprint of Yahoo search advertising on the Web“.

The deal: use BOSS free of charge but as you grow and succeed, we’ll require that you place our search ads on your BOSS-powered search engine. Given Yahoo’s position in the search advertising market, the idea certainly seemed like it was worth a shot.

But apparently Yahoo has come to the conclusion that this model won’t work and is trying a different approach.

Yesterday Yahoo announced that it was willing to let BOSS developers be their own bosses. Under the new terms of service, developers can use whatever platforms they desire to monetize their BOSS-powered products. But they’ll have to pay for it if they do more than a certain threshold of API calls.

According to the BOSS fees page, Yahoo is basing its pricing on ‘units‘, which are used to purchase API calls. Different API calls require different amounts of units. Yahoo provides 30 units per day free of charge, which could represent up to 10,000 API calls per day depending on the type of calls.

It’s not a bad model and it’s probably more convenient for both developers and Yahoo. Dealing with the display of search ads (and possibly revenue sharing) on potentially thousands of sites run by third parties is not nearly as efficient as simply charging for API calls.

There are other benefits to this model as well. Thanks, of course, to the fact that it’s being paid, Yahoo will be able to offer an SLA guaranteeing developers some level of reliability. For the most serious developers who are interested in building something substantial, that should make BOSS a more appealing proposition.

That said, it is hard to ignore the fact that Yahoo has done an about-face. In less than a year, it has gone from trying to “expand the footprint of Yahoo search advertising on the Web” by opening up its search platform to selling ‘units‘ of API calls for 10 cents. This seems to be yet another indication of not only how confused Yahoo is about its identity but also of its lack of confidence in its search advertising product.

That’s a problem but given that Yahoo has already effectively lost the search advertising war, choosing to make developers the BOSS under this new model may prove to be a good move after all.