For a long time, experts have predicted that vertical search engines, which focus on particular subject matter to provide better results, are set to emerge with a bang. But it really hasn’t happened. So far, the most successful vertical search tools haven’t met expectations.

After all, you don’t go to ESPN for a sports search -- Google (Nasdaq: GOOG) still seems like the best place to search for any subject.

The most successful vertical search engines today are associated with commerce sites. You search for books at Amazon, airfares at Expedia, and second-hand anything at eBay.

This isn’t what everyone anticipated, and I think I know the reason why.

When it comes to specialty content, such as books or airfares, online commerce sites can focus on assembling that content and putting together a search engine that focuses on the exact elements someone wanting to buy those things cares about, whether it is book subject, airport schedules and prices, or merchant rating. That search interaction is different from what Google does with a typical content search.

Subject-oriented vertical search engines don’t need to do much of anything differently than Google -- they just need to limit the content they search to what’s pertinent to that subject. If ESPN ever produced a sports search engine, they might very well use the same kind of algorithm Google does, but limit results only to Websites known to be about sports.

That’s why these subject-oriented search engines have never made a mark. There’s nothing about them that distinguishes their search results from Google's. And they are stuck with technology costs just as high as Google’s, because they must figure out how to build a search engine to outdo Google’s ranking algorithm -- albeit over a smaller bit of the content.

Still, there are exceptions. Recently, I saw a new vertical search engine that could be a harbinger of vertical search to come. Truevert is a “green” search engine that limits its results to environmental topics. But rather than building a whole search engine from scratch, Truevert implemented a new semantic search layer on top of Yahoo Search.

This approach avoids the costs that most subject-oriented vertical search engines are saddled with, while still providing a way to best Google and Yahoo Inc. (Nasdaq: YHOO) with better search results, not merely subject-limited search results.

Herb Roitblat, one of the folks behind Truevert, says that they can easily deliver vertical search engines using their technology, on almost any subject.

Time will tell whether this technology is truly the future of vertical search, but I believe that the approach is the right one -- to leverage existing search engines’ investment while implementing a layer above that improves the search results themselves.

Expect to see more companies attempt this approach to vertical search engines, because the real trick to successful vertical search isn’t only better search results; it’s how the vertical search engines make money doing it.

Using an existing search engine, but differentiating the content and the search results, provides a low-cost way to give searchers what they want. That’s an idea that might finally make subject-oriented vertical search an economic success.

— Mike Moran, author of Do It Wrong Quickly, is a speaker and consultant on Internet marketing