The primary models are,
Flat rate PPC:
Flat Rate PPC is where the advertiser and publisher (search engine) agree upon a fixed amount that will be paid for every click. The publisher has a rate card for PPC plan based on several areas of their website. The amount varies as per the content that attracts more or fewer persons.
Bid based PPC:
The advertiser works with other advertisers in a private auction hosted by a publisher. The dealer will notify publisher for the highest amount of bid he is ready to pay. The common method among bid-based PPC websites such as Google Ads is to charge more than the next most important bidder.