Since you asked a rebuilt 3.3 or 3.5 runs between $3700 - $4000 and if the transmission is an automatic they can run upwards of $3000 rebuilt. Considering the value of the car I think having to invest up to $7000 for an engine and transmission would be "expensive", don't you? I know your going to say get a used one out of the wrecking yard, well I would never use a wrecking yard automatic, and would be leary of most engines. Do not forget I had a 733i.