I have a 3G which i brought from the UK, i cracked it and use it on Play pay as you go.
Realistically i don't use it much for calling, but send a lot of SMS, download emails many times a day as well as the use of facebook...... It dosent cost much in data and i only use 30-50zl a month. The main cost is surfing the net which is a little more expensive but realistically not that expensive!
Play is a good carrier and they run a lot of promotions... for some reason for the last year ive had free SMS (not that i know how i got it!)
playmobile.pl/pl/play-karta/startery/index.html
There are other networks but personally i have no experience with them :)
Good luck!