I think you could not really pull a GPUminer on bots - that usually need drivers installed correctly etc and it really slows down system performace - I think it would slow down their computers too much so most of the victims would simply buy new hardware.
However, I'm administering a Computer pool with 80 machines here for work, and thos machines have to stay on at night but theres no one being able to use them, so what I can tell:
CPU: Intel Q6600 -> 4 MHash/s/Machine
GPU: GF 8800 GT -> 25 MHash/s/Machine
that makes roughly between 25 and 30 MHashes per machine, for me it's about 2GH/s, but of course a large scale bot network could do much better.
no one will notice the thousands of extra money being spent on power or anything
good luck with your job
Lol sounds like someone is jealous. This is a great way to utilize PCs that are otherwise wasting power doing nothing.