For monochrome, I use 6000 page cartridges @ ~$150 ($0.025/page) and paper @ ~$30-60/case ($0.006-$0.012/page).
Other things (fuser rollers, electricity, maintenance, depreciation) can factor in, so I ball parked those at $0.015/page
When I bill clients, I usually go with $0.045 per page/face for monochrome. My color printing is $0.215 per page/face.
FYI, I use the term face to allow me to bill appropriately on double sided printing.