HP OfficeJet Pro 9015 printer here. About 18 months old.
According to the printer's reports and the dates on paper reams I print about 100 pages a month. About half our paper use is actually copying various documents.
Ink cartridge sets (large black, regular cyan, magenta, and yellow) are the most economical buy at a local Staples. Still the set now costs over $100.00 USD and usage seemed to go through two sets in a year. Cartridges rarely run out of ink at the same time so overlapping occurs. And stock outages are more frequent.
About a month ago, I signed up for the $6.00 (USD) per month HP ink plan. Costs should be $72.00 / year for ink. (No paper included.)
Received the first set of ink cartridges when the printer reported a low ink cartridge. Full set and the monthly billing began when the first ink cartridge from that set was installed and duly reported to HP by the printer.
I will be keeping a very close eye on ink and paper consumption. I do not use HP paper because it costs about 2 1/2 times as much as standard Staples copy paper which works quite well for our needs. May be using a bit more ink though.
HP as well as the other printer manufacturers are very good at gaming the system when it comes to ink consumption and maximizing the profits therein. I discovered some some months ago that slightly used new ink cartridges (OEM) from a neighbor using the same Part/Number cartridges would not work in my printer.
Unfortunately, it will take probably take a few months to determine if the HP monthly ink plan really saves me money or not.
However, per
@Mandark my next printer is going to be a color laser. And probably some other manufacturer....