Wow, 400 EUR would be a gigantic rip-off!!!
You can go Auschwitz by train, the fastest connection takes you there in four hours and a half, and I'd bet you would not pay more than 30-35 EUR for the ticket (2nd class) - probably less.
When you are there (by the way, the town name is OSWIECIM in Polish), you may take a local guide. The price for an English tour is currently 26 PLN per person which is around 6 (six) EUR.
You may have a look at the English site of the Auschwitz museum:
Do not be afraid to ask more questions and do not let anyone trick you!
Thank you for your answer!!!
I've been checking the timetable to go to Oswiecim and maybe too long just for a day trip.
Being my first time in Poland and with only 3 full days, perhaps the best idea will be visiting only Warsawa.