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.