Q: the cost to replace a transmission on a 2008 bmw x3

If you are interested in a transmission overhaul, or installation of a new (dealer supplied) or professionally rebuilt transmission, YourMechanic professionals in certain locales can assist with that. Please simply inquire based on your locale. As far as the cost, that depends on what exactly you want done. For example, do you want a rebuilt transmission from a vendor? Do you want yours rebuilt? Do you want a brand new OEM transmission from a BMW dealer (best option, by far, but clearly most expensive). Also, in removing and replacing a transmission there are often going to be components, ranging from the transmission mounts to cooling system components, that are not re-usable. The bottom line is a precise estimate will depend on exactly what you want done ad the current condition of some transmission related components on your car. Of course, the mechanic can advise you as to the pros and cons of each and every option. In terms of the cost, rebuilding your existing transmission will be cheapest, and the result will be good as NEW, or better, so long as the mechanic follows every single instruction in the Factory Service Manual. I have rebuilt transmissions using upgraded parts that vendors continuously release as design improvements. That is, once transmissions are released in a model year, often improvements are subsequently made to the design, internal components and materials; ALL of these improvements can and should be incorporated during the process of re-building and will literally make the transmission BETTER than when "new". If you have further questions or concerns, do not hesitate to re-contact YourMechanic.