For starters car manufacturers bet on continuing revenue through parts and servicing for every car they sell so it constitutes a big part of their income. A brand building an image of luxury can very easily charge more and people won’t bat an eye, they’ll think it seems about right, relative to the price it costs to buy one.
Secondly the parts themselves can be more expensive, working on the cars may require more technical skill than on other cars and production may be more limited which makes the cost per part higher. In some cases, usually when a not exclusively luxury brand makes a top spec model, the vehicle itself may be sold at a loss and the manufacturer relies on revenue through servicing to cover the cost of production. It’s a risky move but it can lead to opening up a new market share so it’s been attempted many times by many manufacturers.
Lastly servicing costs do not really reflect on reliability. The higher performance a vehicle is, the harder it is to make it reliable because its component are under bigger stresses. Also the more specialised the construction, the more specialised the mechanics have to be, or procedures are carried out in a non typical fashion, compared to production cars for which many things are more or less standardised. Bad reliability is only when a vehicle which is maintained according to manufacturer instructions and guidelines still suffers failures or mechanical issues. A car having shorter service intervals is not inherently unreliable.
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