This?
I have come across a great site:
banknoty.republika.pl/glowna.html
It covers all Polish notes since they were made in late 18 century.
50 zlotys note from 1925:
General description:
banknoty.republika.pl/rzeczpospolita_polska_1919/BP_waluta_zlotowa/50_zlotych_1_wrzesnia_1929.htm
Prices:
banknoty.republika.pl/ceny/ceny_banknotow_rzeczypospolitej_1919.htm
Three kinds of them, depending on the quality of a note: the better, the more expensive:
from 200 to 2 500,-