To cast a block for a diesel engine requires a different harder metal(s). The high stress & compression can cause weak metals to bend & even break. Then there is a complex injection system along with a turbo/intercooler, & a completely different ecu. The higher torque also means a fat beefy torque converter for a/t trans. Then all that means it's heavier so the suspension of a car will need to be beefed up as well. It's not more expensive to own, it's just the build process is much harder & time consuming.