Let's keep Vienna for comparison, but benchmark it against Warsaw (also substantially more expensive than the rest of the country). According to the data on Numbeo, "you would need around 2,562.64€ (12,036.56zł) in Warsaw to maintain the same standard of life that you can have with 4,000.00€ in Vienna". Making Warsaw only 1.5x cheaper.
Gdańsk is probably in the top 5 most expensive ones (meaning cheaper than Warsaw), more or less at a level you'd expect from a city of that relative size (at roughly 450k inhabitants it's the 6th most populous in the country).
