In this video, a Canon print cartridge is opened up and revealed not to contain the meagre 11.9 ml (0.4 fl oz) of ink it is advertised. The proposed solution is to buy a printer designed to be manually refilled with bottles of ink (such as the featured Epson EcoTank ET-2850), though it has only been tested briefly.
Could possibly weigh the foam thats soaked in ink then wash the foam to remove the ink and dry it out then weight it again and compare. Might not be very accurate but it would be an attempt at closer numbers at least.
Even then you’re supposedly paying for usable ink. I don’t think the ink will all drop down from the foam.
This can be tested still.