My consultancy handles "all things technology" for multiple small businesses (sized 1-50 people, located mostly in South Asia). Anecdotally:
A lot of these folks are happily using mybusinesname.accounts@gmail.com and so-on, but decide to get their own domain for the added veneer of "professionalism".
Many are happy to use Yandex (1000 users with your own domain for free @10GB/inbox. Probably not appealing for some Americans given US-Russia political issues).
Startups/sole entrepreneurs typically come to us after they've purchased a GoDaddy domain and selected the "Business Email" option upon checkout, so they just continue using it.
There's a small sample of companies who are willing to pay for Google Apps because they like Gmail's web UI, and they want the rest of the Apps Suite too.
We also deal with a lot of local big corporations for project work. They're almost always using some kind of in-house corporate email system running Microsoft Exchange Server with a number of different rules like no attachments over 5 MB and other such silliness.
For us to invest time in setting up a mail server for a customer there would have to be a very rare confluence of:
a) I don't want Yandex
b) I don't want to pay for Google Apps / Zoho / other
c) (Optionally) I want to send 500000 marketing/transactional emails per day but I don't want to pay Sendgrid/Mailgun/Mailchimp/SES
To set it up we'd have to establish pricing and SLA. Would I agree that we do it for less than $5/user/month? Nope!
I can't really see a compelling case to do it unless you're a hobbyist with strong philosophical perspectives on this topic. Maybe my point of view is different because of Geography but I doubt things are very different elsewhere in the world as far as cost/effort is concerned.