I have a source telling me that by 200 BC, mercenaries were earning 9 obols per day (here). How does this compare with the average Greek citizen? How does a Medieval European mercenary's earnings compare to that of the average citizens during their time?
In Classical Athens, the average citizen is not the same as the average person. For one thing women and slaves had little cash income. Here is a list of people (mostly citizens) of Athens who were paid a cash income and a rough description of general prices: http://www.tulane.edu/~august/H310/handouts/Finances.htm#Wages%20in%20Athens
As for the 9 obols, a drachma is 6 obols, so it's about 1.5 drachaa, a roughly comparable wage to other laborers and craftsmen.