Generally, the "ISP" is whatever Tier 1 or Tier 2 Network provider is available in the particular datacenter the particular site in question houses their servers, and is preferred by the site in question. Truly massive sites (like Google, Facebook, Microsoft, etc.) will even have their own, private datacenters and (often) dark fiber that they own and use exclusively.
However, an example of a big site (and a company I know well, but won't name), houses the servers for their primary location in the Datacenter.BZ datacenter in Columbus, Ohio. There are several Tier 1/Tier 2 data providers who service that datacenter (have "last-mile connects" into the building), and the company in question prefers to use Verizon Business, so that's who they use as their "ISP."
How much it costs relies on so many variable factors are impossible to even do a hand way guess at (except to say A LOT), but the basic idea is that these companies don't rent a dedicated server or VPS, they rent a rack (or multiple racks) and stuff them with servers and networking gear, and do this at multiple Tier 3/Tier 4 datacenters. They then rent enough bandwidth to service those racks of gear, and power to run them, all of which is extremely pricy due to the high-availability and redundancy demands of these services in a Tier 3/Tier 4 datacenter.