i have an HP K850 13" X 19" 4 color printer as my desktop printer. each of the 4 carts cost near $40 at each replacement. there is a company called CIS INK.
http://www.cisinks.com/ and they make a "bulk outside tank" system for my HP. the kits to replace my carts, cost $49.00. to buy replacement ink, i get 4- 4 oz bottles for $21.00. i of each color. very little modification of existing printer, and your up and running.