charging accounts by resource use -- how to?

[George A. Planansky]

We need to charge users -- or rather their project accounts -- according
to their usage of system resources.  We already do this on a Harris
based system, but we are changing to another machine, installing a network,
getting workstations ... and going over to UNIX.  Negotiations are still
in progress, so the machine(s) and the UNIX are as yet uncertain.
We are aware of AIM's accounting software, but only just.  
I'd appreciate your suggestions or comments, regarding ways and software
and sources, for handling usage charges, on a UNIX network, for upwards
of 100 users and 100 billable projects.  Is this subject discussed in
some particular place I should look?