Orange and Plus announced that they would start prepaid number portabilityon 1st June. The price is 61 PLN (about 15.3 EUR) in Orange and 50 PLN (about 12.5 EUR) in Plus.
It's rather expensive especially if to consider the fact that a new SIM in any network can be bought for 5-10 PLN (of which 80%-100% is credit, some free SMS to notify about new number may also be included).