GoDaddy also has north of 50 million domains registered versus 3 million for Namecheap. If I understand correctly, GoDaddy doesn't make any (or very little) of domain sales, it's the other services like hosting, email, etc that generate the revenue.

How much GoDaddy charges for domains has little to do with their SEO ranking.

They do use it as a loss-leader because most people starting a new site search for domain names over hosting when starting out.