The best site search utility for you will probably depend on your site's technology. If your site already uses a database such as mysql, then there are most likely many possible site search scripts or plug-ins easily available.
Google has a site search offering [google.com] that starts at $100 a year. And there's site search functionality available within Google Analytics, too. But there are many other solutions as well - from free to very intense. Any decent site search utility is going to give you access to this kind of reporting.