They are not expensive at all if you are smart about how you maintain them. The gassers can get quite spendy, with MAFs, oxygen sensors, electric fuel pumps, and other ignition and fuel injection bits that need service/replacement/repair at frequent intervals. When diesels need work, the spending tends to come in bigger chunks but if you keep the car in good shape those big bundles of spending will be few and far between. Just be intelligent about the maintenance you do: use good synthetic oil, quality parts, and make sure the engine is in a proper state of tune. Do timing belts right, set timing properly, etc. If you do all that, the car will never ask you for anything.
The diesels got their troublesome/expensive reputation not from their design, but because so many mechanics bungled them up. Then they started to run badly, break down and wear out. If you take care of it, it will be more reliable, longer-lasting, and cheaper to own than an equivalent gasser.... plus a lot more fun.
