Its easy. You just have to take a look at the financial ledger balance of the company to see their income per month.
Its almost impossible for anyone to find out the exact finances of a company, unless they post a yearly report. People could only estimate how much a specific company makes, and 80% of the time, those estimates are off by a large portion. For example, Hostgator.com hosts over 1 million websites. I can assume they make over $4,000,000 a month, as thats there cheapest plan. However, i'm sure they make about 5-6mil a month easy as they have Dedicated servers, etc.