There doesn't seem to be a common mileage for them to fail at, if they do fail. Failures have happened from 30,000 miles up well past 100,000 miles.

If you get it rebuilt, others have paid in the range of $2000 to $4000. Replacement rebuilt transmissions have been quoted at $5000+

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue