Well a 10 hour flight in a -400 from the west coast of the U.S. to France uses about 40,000 gallons of Jet-A which is sold for around $1.15 a gallon. That's $46,000 including reserves. So that would be approximately $4,500 an hour to keep the plane flying in regards to fuel only.
In the test cell, the JT9D-7AH burned 52 gallons a minute at takeoff power. (Full rated thrust). That was average, we did 4 or 5 different engines to establish a baseline for a data project we were doing.