Cheap fix for atm transmission slipping
You can try just changing the fluid and see if that helps, but once a trans start to slip it's too late. Sorry, there is no cheap fix.
Try to source a used Tranny from a wrecker/wrecking yard and get it installed.
Looking for a Used 3 Series in your area?
CarGurus has 37,596 nationwide 3 Series listings starting at $1,700.
Search BMW 3 Series Questions
BMW 3 Series Experts