When a website pays a certain price to a search engine company, in order to include their website in the search index, and thereby increase their SEO ranking, it is called as a paid inclusion. It is a safer and economical form of search engine advertising. One only needs to pay a single price to have the website listed in a specific search engine, and often under a particular keyword. This makes sure that the site will be updated on the search engines data base more than the non-paid ones.
Nonetheless, you should keep in mind that the algorithm is the final decider, and not the pay. In fact, some of the search engine companies even offer permanent visibility for one year, so that the link can be permanently attached to an established website.
Paid inclusions programs were extremely popular in early 2001, but lost their impact as Google did not approve them as they feared it would prejudice their search results. All the same, Google changed its mind over a period of time. Shortly after Google went public in 2004, Google earned an excess of $23 billion, the other main search engines began to drop their paid inclusions programs so as to catch up. And with time Google seemed to be comfortable with these, raising high concerns for publishers and searchers alike.
In fact, Google is now leading the charge in executing and employing these inclusion programs in particular areas of their business model, in a different form. For instance, Google Flight Search, Google Hotel Finder, etc all have new patterns of paid inclusion programs. This helps in giving better search results, by means of having access to more information. Besides, companies find it a lot easier and simpler to completely control the content of advertisements found on Google.
Thus, it has helped in increasing their quality by leaps and bounds and finding it rather helpful in the search process.