According to the official Central Statistics Office of Poland (GUS) the average monthly gross wage is 3899,78 pln (data from 2015).

The most recent figures, relate specifically to the ‘enterprise sector’ which show that the average monthly gross wage in January 2016 was 4101.36 pln

 

Other unofficial salary comparison sites offer more detailed and often varied information.

Salary Explorer suggests the average monthly salary in Kraków is 7,252 pln.
Whereas Trading Economics, suggests 4066,95 pln as the average monthly wage.