Well, first of all, you'll need the required technology, php in this case, combined with mysql off course. You would have to make a login system, construct a decent search engine etc. etc. But think of this: do you really think people would be interested to pay for your search engine? From what I understand from your post, users will be served with results coming the person that pays most. That would make a louzy engine. Suppose I searched for potatoe and I'd be served with some sort of insurance site in stead of the National Potatoe club. Would I, as a regular user, want that? No! 🙂 So you'll need some sort of staff, people that check whether your paying clients pay for words that actually reflects the content of their site. And hiring staff costs money...
What's more, considering that there are over a 1000 search engines on the web, why would I wanna use yours, if I'd get better results using google?
Btw, search engines like google use spiderbots to search the web and cache all sorts of info: headers, titles, even entire webpages. This costs a lot of server space, thus a lot of money. Also keep in mind that a good search engine needs to have a 100% uptime and needs to be lightning fast. $$$.
Well, If you still want to do this, I'll wish you all the best. Let me know when you're finished 😉