How is the HP Instant Ink savings percentage calculated?

The savings claim of up to 50% is based on the HP Instant Ink Service plan price for 12 months using all pages in the plan without purchasing additional pages, compared to the cost per page of the majority of color inkjet printers under $399 USD. Cost per page comparisons are based on estimated street price and page yield as reported by Gap Intelligence MFP Weekly and IJP Weekly Reports Q3 2013. (Page 2)