The only overhead of the manager is on the server(s) it's installed on.
IT's part of the bundle, and I highly doubt you can save money by cutting it out - I don't believe it's even listed that way. IT's a package deal.
HOWEVER, you can install as much or as little as you want.
At home I run it in unmanaged mode on all my computers.
And as the others have said, if you have more than a small handful of computers, using the management piece is much more effecient. I run it on 2 VM servers - that way the memory and processors can be adjusted as needed at the drop of a hat. And for us, I discovered I needed to add processors and run 3 gig RAM minimum. But we're strange that way - our network takes some heavy management even for the small size of only 370 clients. Partly because we are in 45 offices around this state, partly because our users can't or don't or won't understand safe computing and have decided "it is there, therefore I must click it".
I'll say one thing - TGFSEM/SEP (as opposed to TGIF for example)
Now, SEM has nothing to do with installing firewall, etc. on servers. SEM is the management, SEP is what consists of AV, firewall and so on.
I'm here to say now - I've got the FULL package installed on our servers. I did NOT do that with earlier releases, but with the vERY VERY LATEST, it's running on domain controllers, file and print servers, etc. and NO issues! (at least none that didn't exist BEFORE sep)
It's not interfered with anything, AND you can create an install package for servers that is only AV, and one for other clients that is the full package.
It's VERY tune-able, meaning you can make it all or as little as you want.
IMO, install all of it on all servers, the management on 1 or 2 as you need, and tune and adjust as needed. You can install it all on a server, and if for exmaple the firewall causes issues, go to add/remove programs and modify then choose to remove the firewall. It's that simple.