All this is why (if you do charge back) you simply charge the client by what
the "auditocc" shows...
Then if "THEY" take their processor time to compress their data they get a
little bit of a charge break
but if "THEY" don't take the time to compress their data, they end up paying
more !
Or should I word that as You end up making a bigger profit ;-)
Sure charge them for 20 or 40 GB of stored data, then compress it down to
just 10 GB (or 1 3590 tape)

What I've seen over time is that clients will use the money saved by using
compression to buy more CPU's for their processors thus making backups run
faster at night (by performing compression quicker) AND makes for a better
running system during the day when they have their clients on their box !

 but yet again, it all  boils down to
        It depends ;-)

Dwight