In a particularly frenzied bout of travel in July of 2012 I jumped on and off 6 flights in 6 weeks, crisscrossing the entire continent several times (Barcelona to Berlin, Paris to Budapest, etc).
Total Cost for 6 flights: Less than $200
In rare form I didn’t keep track of these expenses because it was too cheap to worry about (my flights averaged $18 US total). I even bought flights I didn’t use just in case I was in a particular city a few weeks down the road.