Dell AIO 948 printer

We seem to have shocking ink economy from this printer. We are getting no more than 150 pages out of a high capacity black ink cartridge ( genuine Dell ). The printer pop up window will tell us we have 395 pages remaining and then after printing a booklet of 11 pages of plain text ( some pages not even full ) the pages remaining has decreased to 350!! I have tried to alter settings but there is no info on what is the most economic setting to use and how to set it?? Print quality isnt overly important for what we are doing. I tried to switch settings from Auto to Draft but when you choose this option and then go back out of settings it defaults back to Auto. What can we do? We are paying big money for genuine cartridges and the printer is eating them like no tomorrow.......not happy as it has cost us plenty so far......