You don't need to pay for an organised trip to Auschwitz. Catch the PKS-Oswiecim bus which leaves from the lower floor of the bus station behind the railway station. Ask at the ticket office if you can't find it. Pay on the bus (about 18zl for a return ticket). The bus is slightly larger than a minibus and when I went it was driven fast on the bumpy roads, so be warned if you suffer from travel sickness! It picks up and drops off along the route, but it is obvious when you arrive outside the Auschwitz camp gates and the driver announced it on my trip. Your ticket has return times printed on the back and the bus picks up outside the gates where it drops off. Journey takes about 1hr.20mins. Frequency of trip varies but is about twice an hour during the main part of the day. You don't need a guided tour of the camp. Buy a small booklet (c.7zl) and read the comprehensive display boards. Entrance is free. Allow two or three hours for your visit.
Bus station on ul.Bosacka behind railway station.