IMO, the law of supply and demand applies. As long as there is demand for parts there will be supply. Perhaps as demand declines a few suppliers will buy up available used and new parts but I don't think that will happen. I don't think even the most rare of stock X5s will ever appreciate in value. The annual cost of maintaining an X5 is certainly not cheap but it is dramatically less if one is DIYer. I wouldn't own a 20+ year old vehicle if I wasn't able to fix it myself. Labor costs won't be decreasing and have passed point of reasonable for a vehicle that is in the resale range of E53 and E70 X5s many years ago. I bought my X5 new and have not experienced anywhere near the repair costs listed here.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas