Maybe because that's the going rate for a good example, especially with the 2.5ltr engine.Comparing the price with your 520i is like comparing lobster with out of date hoki. One is in demand, the other you have to discount to move it.
Good luck with finding an E30 Coupe for $2k, if you get one with a warrant for that you're doing well.
I get that not everyone likes or wants an E30 (even on Bimmersport) but the prices are where they are for a reason.