Hello Mirek --
You are probably much more an expert in such medieval coinage than we are. Your coin from Poland (Polen) was minted during the reign of King Sigismund I (1506 to 1548).
The coin's denomination, grossus, is sometimes named grosz and groschen. This may be helpful when searching for your coin on the Internet.
Looking through the auction results for this coin, it does not command high value. It is what collectors call a sleeper -- a coin that is undervalued now and likely to grow in value rapidly in the future. Typical catalog values run like this:
worn: $10 US dollars approximate catalog value
average circulated: $15
well preserved: $50
With that very general guidance complete, we can refer you to several web sites which concentrate on the fascinating world of medieval coinage. In order of the likelihood of finding an expert on your coin who will quote a precise price, check out:
- MA Shops