There’s always been an element of that, the cars share so much that it’s always reflected on the cost of parts.
The cost of parts for Z32s have always been excessive for what is a non premium brand, compare what parts cost for say fords of a similar vintage or American cars. You could rebuild a whole Golf for the cost of a set of Z32 exterior trims and the Golf would be worth more. As an example Temp sensor for a z32 £28 for a mk2 golf £6. Admittedly It could be worse, Porsche parts are quite expensive but then it’s a bloody Porsche not the same company that brought you the Micra and the Bluebird.
I think the rise in price to a certain point is good because it makes it more justifiable to actually spend money on them. There was a time not that long ago that a 60k service, especially if you had to pay for labour, would make a Z32 an uneconomical repair. It’s now actually worthwhile to look after the damn things.