Why Are Diesel Cars More Expensive Than Gasoline Ones?

Modern diesel engines rely on some advanced technologies, many of which are more expensive than standard components for gasoline engines.
The most basic characteristic of diesels that allow them to be more efficient than gasoline engines is the compression ratio. While most gasoline engines have compression ratios in the range of 9:1 to 11:1, diesel engines can have 16:1 or even 20:1. The higher the compression ratio, the higher the efficiency, according to the laws of thermodynamics. Unfortunately, higher compression ratios also mean higher stress for the engine internals to cope with. Therefore all the parts of a diesel engine have to be made stronger to withstand those stresses over the lifetime of the engine. This of course means that a diesel engine will cost more in comparison to a gasoline engine.
The vast of majority of diesel engines for passenger cars are turbocharged, in order to produce competitive amounts of power. While a turbocharger gives diesel engines a great flat torque curve and strong power, it increases costs. The cost for the turbo itself is not the end of the story though. Since compressing the air using a turbocharger heats it up, it has to be cooled down again before being fed into the engine. This means the car needs an intercooler and the associated plumbing as well, again more costs.
Another key component of modern diesel engines is a high pressure injection system. These systems, more commonly known as "common rail" systems, pump the fuel up to very high pressures between 1000 and 2000 bars or even more. Because of this, a diesel car has to have a secondary fuel pump to achieve those high pressures, again more costs. Once the pressurized fuel is pumped into the common-rail, it is injected into the cylinders usually by using what is known as a piezo-injector. This is an injector that delivers very precise amounts of diesel using a piezo-crystal. The advantage of these injectors is the very flexible and precise delivery of fuel, which allows very good control over the combustion, allowing good economy and low emissions. The disadvantage is of course that they are expensive.
While high compression ratios are great for efficiency, combustion at high compression ratios also has some unwanted side effects. They tend to cause the formation of nitrogen oxides, or NOx. There are strict limits for NOx emissions in all legislative drive cycles, therefore the car manufacturers must reduce these emissions using exhaust after treatment systems. In comparison to a gasoline car, which usually would have just an ordinary three way catalyst for after treatment, a diesel car would (depending on the size) need in addition a catalyst to reduce NOx emissions. Another emission that diesel engines produce is particles, which also need to be handled by diesel particle filters.
All of these are additional costs that diesel engines have to put up with, and that is why they are more efficient but also more expensive than their gasoline counterparts.