Prices are very similar at biggest registrars. The most popular are: home.pl, nazwa.pl and az.pl. Only first have english version of their site and domain management tool. If you have many .pl domains, you can negotiate. If you need more info about possible negotiations PM me, have wide experience in this matter.
Biggest Polish webhosting companies: http://top100.pl/
Stats about number of registered and active domains: http://statystykidomen.pl/
If you're interested in investment in .pl domains - I have many great names for sale.