Contrary to popular belief on these forums, a GOOD PM will save you money and headaches, not the other way around.
I could go in to all kinds of details with the horror stories I've seen, but I'll let BP do the work for me. I'd challenge you to scroll through the forums here to see all of the self managers who are dealing with nightmare tenants and situations in order to save a few bucks per month on PM. Although it's possible, most new investors don't have the skills, resources or network to effectively self manage.
As a matter of fact, I just posted on another thread by a self manager who is being constantly harrassed by their tenant in retailition for a very minor rent increase. That's a small problem, but what about tenants who are professionals and know the laws better than their landlords? They can smell an opportunity from a mile away.
Look at PM like homeowner's insurance- it seems like you are wasting money until your house burns down, they you'll be happy you "wasted" money on those premiums for years.
Your time is the most valueable asset you have- use it to find deals and make more money, not negotiating with your tenant over lightbulb installations.