The deciding factor for rental prices in Poland is the location of the property. There is a large difference between regions with major cities usually being more expensive than smaller towns and rural areas.
The highest rents are in Warsaw (Warszawa), Cracow (Kraków), Gdansk, Poznan and Wroclaw. For a 2-bedroom apartment in a big city you will have to pay between 1500 and 2000 Zloty per month. In smaller towns, rents are around 800-1500 Zloty for a similar apartment.
Particularly in bigger cities there are plenty of 30m2 single-room-apartments (kawalerka) for around 600 PLN/month, excluding utility bills.
In addition to the fees for an estate agent and a deposit, you will have to pay utility bills for electricity, water, gas and waste disposal.
Owners normally require a deposit of one to three month’s rental cost. You should get the deposit back when moving out, but only if you leave the flat in a good condition.