There are many considerations in the cost of a DNS server, including maintenance and software upgrades that must be done to avoid issues like exploits and downtime.
You'll need to have backups in the event of outages, and you might need to refresh the hardware periodically to provide better responses to your visitors.
All of this effort, time, and money is not core to your business. With the DNSimple Enterprise plan, we take care of all the maintenance, upgrades, backups, and any attacks that might happen.