I would think their profit margins are huge, though you're right, they do spend a ton of money on ads.
IIRC, registrars pay on the order of 25 cents a domain, which they then resell for $8 or more, and GoDaddy is the registrar for something like 1/4 of all .coms. Web hosting and email are very high-margin too.
The registrars pay $7.34 per .com domain to VeriSign. So they are only making 66 cents on that $8 domain. That is why GoDaddy bombards you with add-ons when you register a domain.
Do they seriously only pay 25 cents (or so) per domain?
Why has there not been some disruptive registrar selling .com's for, say, $4? Whatever the operating costs of being a registrar are, it seems like they should be no more than $1/domain-year. So if a registrar dropped their prices to $3 or $4 and grabbed half the market out of it, they'd post huge profits.
Obviously those numbers are just guesses, but I highly doubt that it really costs $9/year to host a domain at scale.