Emissions regulations are the biggest factor in diesel engine price and efficiency, IMO. The current and coming regulations are requiring the addition of many fairly expensive and fuel economy robbing technologies. As you implied, I don't think diesels are viable in the American market yet, except for in large 3/4 and 1/2 ton (1/2 ton to an extent) trucks....and with the aforementioned technologies, the seemingly only advantages to having a diesel, at least at the moment, is for towing abilities, not so much fuel economy and certainly not price.