I feel like there can't be any reason that e30 m3s go for so much money other than people with more money than sense driving up prices. I don't doubt it's a wonderful driver's car but at some point it's pure absurdity. I feel the same about some ultra expensive american cars that go up for auction, as well.
Related to this, there was a very nice looking 1988 m635csi for auction at a Japanese auction house a few months back. Gorgeous metallic purple colour, left hand drive, alpina wheels, 4/b auction grade. Someone paid 1.9m yen for this car (roughly $18k) for this car sight unseen. You'd also have to add on shipping, taxes, compliance, etc bringing the cost up near $25k-$30k easily for something that probably needs thousands in maintenance.
At that point i basically realized that monied enthusiasts have hard ons for old M cars are willing to pay out the nose to have something that will ultimately sit in their garages not being driven.
Honestly, if you've never driven one (or an E30 at all), you wouldn't understand.