> Typical salary for a senior dev is around 20-30k PLN per month

Typical, yes, though it's possible to earn way more than that. Not that 20-30k PLN per month is bad, the average Polish salary is perhaps around 9k and the median around 7k.

20-30k PLN goes a long way in Poland. Some seven years ago, I was spending around 7k PLN a month, living in the beautiful Warsaw old town, 50 metres from Kolumna Zygmunta, eating out all the time, and generally felt I was living like a king. Good times!