Transmission fluid is the most commonly deferred fluid service on the average car, and the consequences of indefinitely deferred service are among the most expensive mechanical repairs an owner will face. An automatic transmission rebuild or replacement costs $3,000 to $6,000 on a mainstream car and significantly more on European or luxury vehicles. A transmission fluid service costs $100 to $250. The return on the maintenance investment, expressed in avoided repair costs, is among the highest available in automotive ownership.