My car's reverse gear will not engage. I just changed the transmission oil with full synthetic fluid. The problem still exist. Is this the sign of a transmission job? If so, what is the average cost to repair the transmission?

Thanks for the inputs.

__________________
94 E320 Silver 210000 miles
94 740il Pewter Green 97000 miles- Retired
05 S430 4Matic
07 Lexus IS250