e28s in general and M5s in particular have seen a pretty dramatic upswing in value over the last couple of years.
This 535i sold on BaT a few weeks ago for $50,000.00.
https://bringatrailer.com/listing/1988-bmw-535-4/
I bought my M5 10 years ago when prices were still 'reasonable' and I wouldn't take less than $50k today.
