These are the 80ml cartridges - they were formerly priced at $92.50 and the price has now dropped to $79.50. That's less than $ 1 a ml and it means that your printing costs are on the way down - and your profit is on the way up. I recently did a run of self-border boudoir-size prints on Ilford A4 Smooth Pearl paper at $ 70 for a box of 100 and the base cost per print worked out to $ 1.69, less the cost of the coffee and biscuit needed to operate the printer. ( I'm the printer.)
At these prices there is no need to mix your own ink from the contents of your child's watercolour paint set and try to pour it into the back of the machine - you can afford to use the correct factory K3 ink.