hmmm good question. Maybe I should write one.

But in my own experience, as a 25+ years IT sysadmin, running many email domains and servers, the cost is insignificant, is practically the cost of buying a domain and keeping a simple PC online 24/7. The software, depends on what do you want to achieve it could be even ZERO cost, free. If you want advanced solutions for business, then yes, it can involve some costs.

But nowadays, anybody can run their own email server/domain at home, for personal private use, with full control of the software, hardware and content.