Anyone here running a domain registrar business?

Depends on the volume. But it’s mostly under the 80k required capital.

There are even some open source implementations for the backends.

Most expensive points are the required employees and the signup fees (and prepaid) for other registry’s like .xyz

DNS via cloudns for example is around 10ct per Zone per Month via there DDoD Protected package.

Email Infrastructure (for renewal etc.) can be acquired over different services like postmark.

Validation of contact data can be expensive too. However maybe something like nominatim could do the trick.

Another thing is infrastructure for Whois/RDAP.

Then you need standard things like Whois privacy (there is a document by icann for requirements).

I would guess that 8-10k per year without the icann / tld fees, is a good starting point.

Even though it's discouraged I want to say thanks for these excellent points, that's very helpful!