Which printer is the most cost effecient

If you don't need color, a laser printer. The HP 2200 series are a great bargain. They are pretty cheap (as they are used and a bit old) but will run till the sun blows up it seems. I have used a few that were from a tax office, printing out thousands of pages weekly, and they've been running fine for me and some friends for years after that. Still using it 5 years after it got retired from active use for tax printing.