"I think you are wrong. Both diesel and petrol engines are built to the same tolerances and, though engine management for diesels is even more sophisticated than petrol, the impact on cost is negligible."
Horse... well, feathers.
Why "think" when you can "know"? Google is your friend. Here, let's look at example parts costs:
==
formatting link
Mahle Piston Set - .50mm Volkswagen Jetta GLI Piston Set: List price, $311.03, "our price" $259.19
Mahle Piston Set - .50mm Turbo Volkswagen Jetta Diesel Piston Set: List price, $504.68, "our price" $420.56
==
formatting link
Volkswagen Golf III GL 4 Cyl Cylinder Head Gasket, Reinz: Your Price $21.43
Volkswagen Jetta Diesel Cylinder Head Gasket, Reinz: Your Price $29.92
== These are typical, and reflect significant design, construction, material and, yes, tolerance differences (especially in the fuel injection system, where pressures can exceed 2000 psi even in indirect-injection configurations). If you'd ever seen even an economy automotive diesel engine stripped down, you'd have an appreciation for this. And, worldwide, diesel engines are very popular; your conjecture about relative volumes is incorrect, though certainly they hold in the U.S.
If you'd look instead of "thinking", you'd find nuggets such as "The cylinder head for the [VW] turbo diesel uses a different gasket [vs. the gas model] and is cast from a special stronger alloy. Different materials are also used for the turbo diesel cylinder head's valves, valve seats, and combustion pre-chambers."
formatting link
and "Although theforged-steel connecting rods are similar to those used in thespark-ignition engines, the pistons are of far more robust dimensionsand are totally different in design, since they must be capable ofattaining very high compression pressures and withstanding the loads ofcompression ignition. As on other Volkswagen engines, full-floatingpiston pins are secured by circlips. The connecting rods for the turbodiesel engine have greater piston pin clearances, however, and thepiston skirts are notched for clearance with the piston-cooling oiljets."
formatting link
and "Dieselengines and high performance gasoline engines feature an oil coolerattached to the filter housing through which engine coolant circulatesto help moderate oil temperature."
formatting link
You are correct, however, regarding the lubricity of diesel fuel and the whole-life lower cost of diesel engines, at least mechanically (although as I've noted from my experience, the rest of the car can be just as crappy and have just as short a life as a gasoline model!). Much depends, of course, on the relative costs of fuel. That varies, even in a given location. In the '70s diesel fuel was usually considerably cheaper than gasoline in the U.S. Today it generally isn't. In Europe today, the relative prices of the two fuels depends mostly on regional tax policies, since taxes are a much larger component of fuel costs there than in the U.S. (where they're still very high compared to, say, oil company profits per gallon).
And you neglect the fact that in conventional (non-hybrid) installations, diesel engines need all the stuff a Prius doesn't: alternator, power steering pump, belts and followers, clutch or torque converter, transmission, shifter and linkage, and so on-- all of which needs maintenance, the occasional repair, and periodic replacement of significant components. By comparison, the hybrids need their little planetary-gear power-split gizmo, two electric motors, and a battery.