In recent years, there has been a rising concern about the policy of major search engines. The concern comes from search bias, which refers to the ranking of the results of a keyword search on the basis of some other principle than the sheer relevance. This search bias is also named as search non-neutrality. In this paper, we analyse one non-neutral behaviour, that is, a behaviour that results in a search bias: the payment by content providers to the search engine (a.k.a. side payment) in order to improve the chances to be located and accessed by a user. A game theory-based model is presented where a search engine and two content providers interact strategically, while the aggregated behaviour of users is modelled by a demand function. The ...