Search Engine scraper

I am looking for to develop or information on how to develop a software product that will use the search engines to get information for keyword research. There are many companies that do this such as: Market Samurai, WordTracker, Keyword Discovery, SpyFu and many many more.? I know that you can purchase and API key from Google and use their search engine to run searches. But you have to pay for the API key based on searches. I know some of the above keyword tools you pay a one time fee and that is it. So, they are using something different than the API code.? This is what I am looking for.? Thank You