As per Google’s methodology, crawling is an approach in which the Googlebot unearths new and updated pages and its related content and adds the same to the Google Index.
They designate this job to a gamut of computers and assign them the task to go and fetch (or ‘crawl’) through billions of pages on the web. This initiation is led by a program named the Googlebot (in other words a robot, bot or spider). The Googlebot implements an algorithmic process which includes the computer programs determining which sites to crawl, its frequency and how many pages to crawl from a particular site, all of which are subjective to the algorithmic structure set by Google.
The process is commenced with a list of web page URLs that are acquired from previous crawling data and amplified with Sitemap data put forth by the webmasters. Now, as the Googlebot browses through each of these websites it detects links on every page and affixes them to the rest of their list of pages. New sites, revisions to existing ones and dead links are taken note of and updated via Google index.
There are no monetary means that can entice Google to crawl one website more than the other or for that matter, even more frequently as they keep the revenue generating bifurcation of their business such as AdWords afar from the search side of the business.