charging for resource usage

[George A. Planansky]

How is charging, of users and projects, for resource usage (cpu time,
disk space, ...) generally done in Unix?  I gather that some use the
sa command plus scripts to do this.  Is there a source of useful
scripts out there?  Is there something better?  This would apply to
a science/engineering research group of some 50 users, and say 50
projects, on a supermini plus workstations and other paraphenelia.
I'm talking about tracking resource usage, and not the more philosophical
question of who should be charged for what, and why.