Much of the cost here comes from compliance with the ICANN gTLD program structure, not from running the underlying technical infrastructure (which is not limited to DNS - you also need EPP/RDAP/etc). See https://www.icann.org/en/registry-agreements for (hundred+ page) documents outlining registry responsibilities. Registries can outsource some of this to an ICANN-accredited "registry service provider", but should expect to pay upwards of hundreds of thousands of dollars yearly for the privilege.