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
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).