In the Curves pallet, below the graph, are the input and output values for each control point that is plotted on the curve. But you don't see these values unless you have a point selected.
Using Curves to set white balance with the gray eyedropper tool only puts one control point on each channel. And that point is moved left or right along the center horizontal axis. So the Output values stays at the center point, or 127.
After you do the white balance, go to one of the channels and click on the control point (little red square) on the graph, and the input/output values will appear. You can change the control point by dragging it around, or you can simply type in new values.