Exactly. I run a registrar price comparison service, and a lot of domain extensions are basically sold ‘at cost’. The catch is that a) a registrar will sell some tlds with a tiny/no margin, but charge a healthy extra for other tlds (like a ‘loss leader) and/or b) get you on the renewals (or c) keep increasing fees once their number of customers is high enough and hope most of them won’t notice for some time, double or triple of regular fees is not uncommon!).

So even though margins are low, as a customer it still makes sense to shop around.

Ps https://dot.bs is the service I run to compare tld registration and renewal prices