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!
I guess you've already traveled to Warsaw. Asuchwitz is far, but Treblinka is not so far from Warsaw....