My understanding is that Netflix just pays a fixed, pre-agreed amount up front (either paying a licensing fee for existing content, or funding the production of their own Netflix originals). If lots of people view a show, Netflix takes that data into account when deciding whether or not to renew the the licensing contract (in the case of existing shows) or order a new season (in the case of Netflix's original productions).
Because Netflix pays for their content up front, they have to take a bit of a gamble. (Maybe they spend a bunch of money for a new Coen Brothers film, but nobody watches it, so they take a loss on that project. Or, as was the case in 2008, maybe TV networks grossly under-estimate the value of their catalog of old shows, so Netflix gets to pay peanuts for the content that serves as the bread and butter.)
Because Netflix pays for their content up front, they have to take a bit of a gamble. (Maybe they spend a bunch of money for a new Coen Brothers film, but nobody watches it, so they take a loss on that project. Or, as was the case in 2008, maybe TV networks grossly under-estimate the value of their catalog of old shows, so Netflix gets to pay peanuts for the content that serves as the bread and butter.)