I’m a Seaver fan, but damn! When did his RC start commanding Koufax prices?
 

Is this a somewhat recent development? Surprising (to me) to see where his ‘67 Topps RC is at price-wise. That being said, I’d love to pick one up :)

Well, it is a short-printed high number in 1967. So it's always going to be expensive for that reason. If you look at the PSA 8 prices, it looks like they spiked during the 2016 bubble from ~$900 to ~$2500, then softened, and are now approaching that peak again.
https://www.psacard.com/auctionprice...ues/187370#g=8

I recently used an ebay 15% coupon code to grab a pretty nice PSA 5. Even after using the discount to take nearly $100 off, it still cost over 5 bills. Ouch.