Auction prices drive retail prices of all cars.
Used caddies (mainly FWD DeVilles, Sevilles, Eldos) have some of the worst resale values out there. Reliability is a major issue with them. Also very expensive to fix. Ever put a starter in a Northstar? How about a head gasket job? They are throw away cars.
I’m not a GP or Mustang fan either, but I’d take either one of them all day before a used Cadillac (with the exception of an Escalade/EXT).