If this function of your ISP costs less than 1 FTE per 10,000 dialups or 1,000 T1's or 100 T3's, then your ISP is a slacker and probably a magnet for professional spammers as well.
... you're offering very definitive figures/labeling, and I'm curious as to what you are basing your calculations/labels on, and what the linearity of the scaling is in your opinion.
Your own experience at MAPS? At MFN? Wishful thinking?
those numbers are very round. i've seen folks do 1 FTE per 50,000 dialup users and get away with it, but that person was VERY busy. that ratio only works if the rest of the system is designed to repel the professional spammers, i.e., full ANI with filtering, full verification of credit cards (charge and refund before opening the account), nonrefundable deposit if terminated for spamming, and instant termination even at 4AM on sunday morning, ~30 hours or more before the account manager or any other manager could give approval.
Personally, I'd much rather try to justify a FTE for 1000 T-1s than I would for 10,000 dialup users.
like i said, the numbers were very round. as long as you understand that there IS a ratio and that the cost of dealing with outbound traffic does not end at the demarc point where it's handed to a peer or transit, then what the actual nonzero "abuse desk" costs actually are is a detail. this seems like something isp/c or cix should do a survey on.