It really depends on how comfortable you are at Linux systems administration, and how ambitious you want your mail server to be. If you're going to have to rely on any (really, any) of the guides or howtos online, then expect to budget in many hours of problem-solving and knob turning.
We currently spend very little time each month on ongoing maintenance for our mail server, and we have a stack that includes good spam & antivirus systems, webmail, easy user administration, and hourly offsite backups.
But, I've got probably over a hundred hours of my time sunk into all of that, and there's still a lot more I'd like to do to the server.
So, from a billable hour standpoint, you probably can't justify hosting your own mail versus Google Apps (or just about anything else), unless you're specifically looking for functionality that Google doesn't provide.
We currently spend very little time each month on ongoing maintenance for our mail server, and we have a stack that includes good spam & antivirus systems, webmail, easy user administration, and hourly offsite backups.
But, I've got probably over a hundred hours of my time sunk into all of that, and there's still a lot more I'd like to do to the server.
So, from a billable hour standpoint, you probably can't justify hosting your own mail versus Google Apps (or just about anything else), unless you're specifically looking for functionality that Google doesn't provide.