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.
I recommend EuroDNS.com for .PL, the more when you own many TLDs - with EuroDNS you have a chance to manage them all in 1 account - it really makes things simple When you register higher volumes, get a contact with sales team and you will get very competitive pricing.