I did some cost analysis on this early this year and from what I could tell, at least with the mgmt company I was talking to, it was close to cost neutral when you figure in EVERYTHING(and that list is long), but I was talking to a very respected mgmt company in the city I am in.
However my personal opinion on the matter mirrors most guys on here, hire a chief pilot and do it yourself. Pay him an extra 20-30k and you will get better service. Most of the fleet discount stuff you can get on your own by signing up for various fuel cards and what not. I didnt notice much difference in price between the fuel costs for the mgmt company vs. on our own with a couple of different fuel cards. Same with everything else. Plus you get some guys that will work harder for you because they have a stake in the operation. With a mgmt company hiring a couple pilots for you, they dont have a stake and thus dont necessarily care how things really run.