No reverse ( step-tronic automatic transmission )

What is your question?

Went to back up and heard a soft clunk. Reverse worked with substantial slippage. Has gotten steadily worse over the course of the day and when I got home , it didn't work at all. Tranny works great in every other gear. Unable to check fluid as I thing the dipstick is on the tranny. TSI transmission repair said it could be some kind of DRUM that only affects reverse gear and has to be ordered from Germany for about $ 5000 installed but they didn't diagnosis yet. I have an appt. this afternoon for a diagnosis at Sunberg Automotive in Burnsville, Mn. What are the BEST and WORST case scenarios ? Thank you ( 124k miles )