Theory: conversion funnel is a simple funnel. Reality: there's a variety of channels interacting and sales/lead are never or rarely a single channel linear funnel.
Sometimes, requires hypothesis on conversion ratios.
What do you pay?
» Cost per 1000 (CPM), click (CPC), action (CPA), lead (CPL), signup, install, sale, etc.
Who do you actually pay?
» Attribution to... last click, first click, linear, position based, time decay, data-driven.