Site Search Engine

A client of mine wants me to add a site search field to his site. Not a bad idea considering the amount of content. He doesn’t want to pay an arm and a leg for it. Which is understandable. And he really doesn’t want to include anything that will take visitors to another site. I thought about using Google, but he doesn’t want any ads. What is a safe, cheap and easy alternative, besides CTRL + F? Something that a newbie can put together with some good tutorials. Also, what open source alternatives? If anyone is using something like a described, I’d really like to hear from you.