However....backup, anti-virus, spam filtering, and a DR solution drives up the cost very quickly.
The marginal cost of backup and DR when you're *already* doing those things for an on-prem server environment is pretty close to zero, and if you're already virtualized and have a virtual-oriented backup software you probably already have DR integrated into your backup. AV and anti-spam are almost always done best these days by a third party service and the good ones do both anyway.
From the numbers I've run, it usually is cheaper to do it on prem above about 50 users with a 3 year benchmark. If you time the upgrade right, you can probably get 5 years out of it without falling more than a rev behind and cut the 50 user number way down.
It's pretty obvious Microsoft is heading subscription-only for everything. Since 2013, Exchange has lost much of its GUI which I think has been a way to scare on-prem admins away. They will ultimately either price on prem high enough that only a few compliance/security focused large organizations will consider it or support hybrid only (meaning you're paying for O365, used or not).
Cloud is about permanent vendor-lock in and rent-seeking, not economics. The marginal cost of a 5-9s commercial data center for hosting cloud services is greater than the marginal savings to users, which is why hosted systems always end up being so expensive unless you're doing something really trivial like a static web site.
You guys are somethign else. You bashed WIndows NT and then called Windows Server for years for not having a CLI. Hey look at Unix we do not have to sit in front of the server to admin it etc. Now MS has powershell and it is BEH WHERE IS THE GUI?! Exchange is a very complex project because many organizations have complex uses. If your admin can't handle scripting and commands then he is incompetent. If you are paying someone +$75,000 a year he or she should be a master at that price and have years of experience.