Yes, it is likely to be a $50 part, but unfortunately that $50 part is a pressure switch, which live inside your Auto box.
This pressure switch directs which 'vein' the box fluid goes down, each vein is a different gear.
So the box is going to need to come out, stripped down, 1 minute to remove the old switch, 1 minute to get in the new switch, put back together and bolt back to the car. So that $50 part is going to need about $1000 mechanic job on it, which is a real pain.
In the UK, as there is so many E36's around, it a lot cheaper and easier to just get a new box.