BMW will not reverse

I have a 2000 BMW 323i with 94,000 miles, automatic. Most of the miles have been highway miles. Recently, the car does not reverse. I would put shift it to reverse and the engine just races with no movement. The car moves forward just fine, and there does not seem to be any other problems with the car. The last oil change that I had showed that my fluids were fine, even though it was not reversing. What could be the cause of this? What would be the estimated cost to fix it? Thanks!