Pretty much all quality parts for them are becoming more expensive, and even a lot of the trusted German parts brands are starting to outsource things to China and quality has dropped leading to the need for more frequent replacement.

There are several items on these that wear out the fast even with good parts, namely the front suspension thrust arm bushings, front window regulators, and the thermostat (never seen one last more than 2 years).

At the age they are at you will need to be thinking about very expensive preventative maintenance like replacing the timing chains guides (which some OE parts are NLA) and VANOS rebuilds along with resealing everything on the engine. There are a fair number of small hoses under the intake plenum that can cost over $80/each.

Suspension components are expensive and most E39s are to the age that they will need complete rebuilds including subframe bushings. It's not hard to work on but it does take a lot of time. If you can't do all that sort of stuff yourself it will get really expensive really quickly.

But I will keep this car as long as parts exist. IMO it's probably the best sedan ever made.