power sterring on 2004 BMW 325i

I'm having a difficult time turning the steering wheel. I thought the power steering fluid was out, but I checkd (ATF) and it has enough in it. What else can be causing this? Is is the pump? If so, how much does that cost? Does it have to be a "BMW pump"

Asked by for the 2004 BMW 325i
Make sure it was the power steering reservoir you had checked the fluid level in. Make sure the fan belt tension is correct (belt is not loose). With the engine running get a friend to turn the steering wheel from side to side and look for movement of fluid in the reservoir. It could be a pump failure, you can get an idea of price of a new pump by looking at www.rockauto.com or any of the on line parts stores. But I hate replacing parts without proving a part is bad.