Who prints pages with only 5% coverage? Epson quote 6,000 pages for the EPL5900 cartridges. In real life I get just over 3,000. So 2p per page is probably about right for your HP printer.
One tip to gauge toner use if you have digital postal scale: weigh the empty cartridge when you change it for a new one. Print 100 pages and weigh it again.