Older BMWs, including our E9s, have a limited audience; auctions are not their domain, and the very best sell privately to a knowledgeable audience.
I know of a Bat that recently sold for $650k and one that sold for $300k. Another sold a few years ago for $850k and everything in between.
Forgive me for saying this, but the "general" audience for the lightweight is not as large as the 2.7RS or as knowledgeable; yes there are a few CSL experts amongst us here, but not much more beyond.
I guarantee you, go back 10 -15 years, and these same conversations happened: "Why is a CSL 1/4 - 1/3rd of a 2.7RS?". Then, around Covid time and just before, our CSLs became 1/2 of a 2.7 RS of equivalent condition and spec.
Unfortunately, the market got exhausted on the supply and demand curve; suddenly everybody was selling their E9s and CSLs and dealers jumped on the bandwagon, the supply has overtaken the demand for now.
Today, the youngsters want E30, E46, etc, the very best E9s will always do well, but the gap between a great E9 and a 2.7RS will always be there, just supply/demand of each at particular times will it be 1/3 - 1/4 or 1/2 ? At times any of these......... but equivalent - I think not - believe me, I wish it were not the case.